Jessie J - The girl from Essex who plans to take the pop world by storm

She's written songs for Alicia Keys and is touted as the world's best singer by Justin Timberlake – and she's still only 22. Elisa Bray meets Jessie J, hotly tipped to be the chart sensation of 2011

If there's one name you'll be hearing a lot of in pop in the coming months, it's Jessie J. She is longlisted on the BBC Sound of 2011 poll, and is tipped to be a Brits Critics Choice nominee. Justin Timberlake has hailed her the best singer in the world right now, she has penned tracks for Alicia Keys and wrote Miley Cyrus's hit "Party in the USA". Her recent London gigs have been attended by celeb fans N-Dubz and JLS, with huge throngs left outside in the cold. And all this by the age of 22, before the release of her first single. With her forthcoming album of stomping R'n'B power chart hits in the waiting, Jessie J is an unstoppable runaway train.

Prior to our meeting, I am told that Jessie J is a character – and from her punchy expletive-filled songs I am expecting a loud, tall (she's 5ft 9") girl from Essex. But Jessie J is not on form. She's suffering from a nasty virus, and emerges, striking with her tall, slim frame, raven-fringed bob and Cleopatra-thick eye liner, carrying a KFC bag to throw up into. "I'm so sorry, such bad timing," she says, with genuine regret. "Great first impression. But I am human. I suppose everyone gets ill." I'm just hoping she doesn't have to use that sick bag.

That Jessie J insists on continuing our meeting shows that there are no diva tendencies here, but, instead, a serious attitude to her career. It's how Jessie J – born Jessica Cornish – has got this far. Ever since she was a child, Cornish wanted to be in the music industry. Her first words as a baby were "jam hot", the lyrics to 1990 hit "Dub Be Good To Me", which came out the year after she was born in Essex, and she was soon acting in Nickelodeon commercials. As a child, she was "a little show-off". "I've kind of looked like this since I was born – I've had this haircut since I had hair. I was always very bubbly, always thought I was the funny one, and was a bit of a tomboy."

She wasn't much like her older sisters, who, at five and seven years older than her, were both head girls at school. Unlike her academic sisters, Cornish was "never really that good at anything". "At school they were like 'oh, you're a Cornish girl' and they kind of expected me to be the same as my sisters. Give me something to draw or an outfit to pick for someone, or hair, make-up, acting, write a song, I'm fine with it, but anything to do with sums – it was never my thing." She adds confidently: "But I never based my intelligence on my exam results."

She was good at something, though – singing. So good, in fact, that she was banished from the school choir, for being too loud. "I was in it for a day and some of the adults were moaning that their kids were upset that I was too good. I was 11. Can you imagine? I was heartbroken."

Her musical tastes were shaped by the soul funk music she grew up with: Bob Marley, Aretha Franklin, D.Train, Michael Jackson, and Prince. If she had to pick a musical idol it'd be between Whitney Houston and Mariah Carey. As a child she was always singing, into a hairbrush in front of the mirror. She and her sisters had a band, The Three Cornish Pasties, in which Hannah played piano, Rachel played trombone and precocious Jessie was the singer.

When she was 10, she landed a two-year part in the West End musical Whistle Down the Wind. "I think it was that that made me grow up really fast because I was going through school to do a show every night. It made me realise that I could do what I love and be paid for it."

At weekends, the teenager took dance classes and eventually went to the Brit school to do musical theatre, where her contemporaries were Adele, Katy B and Rox and where she joined a girl group, Soul Deep.

There's a distinctly non-British confidence about the way Cornish talks about her achievements and it's refreshing. At 16, she was invited by the director of Evita to audition. She says she only just missed out on the role for being too young. Talking about meeting the stars in LA, she says: "When I met Alicia Keys or Justin or whoever, I wanted them to feel I was making an impression on them, too." And the global hit she wrote for Cyrus? "It was the most amazing way to give me a platform and it's not the usual way to be known – 21-year-old writing a US hit. It won pop song of the year 2009, and you can't really ask for much more to be honest. I'm 22 now, but I forget."

She recalls the advice Timberlake gave her in the studio: "He said 'make sure you play this to win, don't do it for the fame or success, do it because you love it and want to make a change', and that's how I've always lived my career. I've got so many people who I admire, like Kelly Rowland saying 'it's amazing how the pressure builds', but I'm like a little girl with a dream."

It wasn't just having older sisters, and early acting jobs that forced her to grow up fast. Aged 11, Cornish was diagnosed with an irregular heartbeat, and was treated at Great Ormond Street Hospital. "It wasn't easy. When I was younger I couldn't really be as free as other kids. I always had to take it slow otherwise I'd end up in hospital. As I've got older I deal with it better. It doesn't affect me every day but it does bother me when I'm tired. I try to make sure I get my sleep."

As a result of her condition, cigarettes and alcohol are off the agenda. "It made me realise that life is too short to intoxicate my body with booze and drugs. I never got into that dark side of the world like so many people do." She adds with a smile: "I'll have the odd glass of mulled wine at Christmas."

At 18, she suffered a minor stroke, a fact she recounts matter-of-fact ly. "More than anything it shook me into a place where it doesn't matter how old you are, you have to get on with life and not take it for granted. It's helped me a lot on how I am as a person and my outlook on life."

When she was just 10 years old, a ward mate of hers, a little boy, died. It's the subject of "Big White Room" on her album, the first song she ever wrote, aged 17. She recalls waking up in the middle of the night and seeing him praying, and her mother explaining that he was having an operation the next day and was asking God to save him. "He died the next day so I said to my mum 'but God didn't save him'. I was so angry and it really confused me. I always wanted to write a song about the experience, but I knew I had to be of an age where it wasn't tacky or depressing and had a lightness to it." She prefers the song with just an acoustic guitar; stripped down and bare.

It's the title song of her album Who You Are of which she is proudest. It's the song that draws the most messages from fans, on YouTube, Twitter and Facebook. "The other day I had a girl message me saying 'I was ready to take my life and then I heard "Who You Are"'. That pressure is beautiful but scary at the same time. I want to be a positive role model for young people. I always say that I'm half-artist, half-therapist", she laughs. She wrote the song at the end of a lonely three-month trip to Los Angeles when she was 20, having been shunted from studio to studio with various producers. "I'm very much someone that lives to be happy. It's not just about the parties and I know so-and-so – I'm not that girl. So I looked in the mirror and started to cry and said 'who am I?' Music is my therapy."

She ascribes the size of the crowds gathering at her gigs to the questions surrounding so many pop singers and the authenticity of their vocals. Cornish, by contrast, is not just a performer but a singer, and everything is live. I know she can sing from the way she bursts into little flourishes of song throughout our meeting. "I know a lot of people want to see me live to see if it's real. I have spent a lot of time training my voice and making sure that when I go to the studio I'm not pitchy and I'm not out of tune and I'm quick. If I'm a singer I have to be good at singing. Live for me is my time to go: 'this is what I'm good at'. And it's nice that a lot of people want to see it."

How does she cope with the pressure of being tipped before she's even released a single? "I kind of like the pressure", she says. "I love it." Does she fear that she could lose herself with the fame that's tipped for her? "I don't have any yes-men around me. I have friends that want to come round play Wii, eat spaghetti Bolognese, and talk to me about stuff other than music. I've never been that girl and I've got a great family and friends. It's important to stay grounded."

Does she have a stylist? "No. If someone did my makeup and my hair, and dressed me and wrote my songs I'm just being a dummy. If someone dressed me, I wouldn't feel like me." She has her personality stamped all over her – she shows me her tattoos. One, "Stand Up", a song title, on her wrist, a lyric from "Who You Are" on her neck an upside down treble and bass clef forming a heart ("I wanted something that says I love music"), and a stickman, which she had done when she was stressed out in LA ("It's something I can look at and go 'I need to chill out'".)

She has just a couple of goals up her sleeve: to win best newcomer at the Brits; to write a musical that goes to the West End and Broadway and to have her own perfume and clothing lines. Above all, she wants to take her music around the world. "I want to be at the top. I want to be a credible artist, not just someone here today and gone tomorrow. You're not going to get rid of me."

"Do It Like a Dude" is out on 3 January The album 'Who You Are' is out in March

Five more set to be the sound of 2011

Yuck

The north London indie band's reflective guitar pop is influenced by American bands of the late 1980s and early 1990s such as Dinosaur Jr, and Sparklehorse, combined with British bands like Magazine and Teenage Fanclub.

Esben & the Witch

The Brighton trio, named after a Danish fairytale, signed to Matador in August and their first release for the label, "Marching Song", sold out instantly. They toured with Foals and the Big Pink this year.

Clare Maguire

Two years ago, the 23-year-old (above) signed an eye-watering $2m deal with Universal after a fierce label chase. It's her voice, which melds the blues power of Janis Joplin and the sweetness of country singer Stevie Nicks, that drew the many offers. She played her first London headline gig last month, attended by Hurts.

James Blake

The 21-year-old dubstep artist (below) has already proven himself a creator of intelligent urban music with his spliced R'n'B, hip-hop samples and piano. His new single, a cover of Feist and Chilly Gonzales's "Limit To Your Love", was recorded and produced from his London flat.

Jamie Woon

The 27-year-old Brit-School graduate is another dubstep artist, with music in his blood (his folk singer mother provided backing vocals for Stock Aitken Waterman and Björk). His new single "Night Air" is garnering the attention – it's on the playlist at ASDA FM.