Nellie McKay has emerged from the barrooms of New York with a catalogue of spiky, sassy songs dripping with 1950s influences. But success is bringing its own problems and tedious promotional demands are getting in the way of the two things she loves best - songwriting and changing the world. She talks to Jim Carroll

There are at least two Nellie McKays out there in the big bad world. The first one grins at you from the cover of her Get Away From Me début album. She's a 19-year-old, gee-whizz musical starlet in a cute, red jacket, peppering you with songs that mix Stephen Sondheim satire with Cole Porter witticisms, in a style that tips the fedora to both Doris Day and Eminem.

It's 1950s pop with 21st-century things to say about American foreign policy, stalkers and animal testing. Bright, breezy, smart and a whole lot of fun, there was so much sass here that they had to roll out a double album to hold it all.

That, however, is not the Nellie who picks up the phone. The real McKay probably sighs a lot more than her golly-gee caricature would ever do. She is the one, after all, who has to hang around on her day off to pick up the pieces and say yes or no to merchandise designs, while her doppelganger gets to twiddle a parasol over her head and saunter arm-in-arm with Cary Grant down Madison Avenue without a care in the world.

As McKay is quickly finding out, finding new audiences for such spiky odes as Won't U Please B Nice or the biting satire I Wanna Get Married, on the joys of wedded bliss, comes with a price. "I'm ready to move on," she says, "but I can't because I have to promote the album. I'm so tired of the album at this point, because I see so much in it that I would have done differently. Of course, it's nice that so many like it and have bought it and have had such a great reaction to it. But I'm ready to move on even though I don't know what to.

"In terms of songwriting, some of the songs on the album are less than perfect. I know, I know, you shouldn't obsess about that kind of thing, but I can't help it - and I do! I just hope everyone else realises that you can't promote the same album for five years."

The stage may seem set for an almighty moan, but McKay seems more puzzled and bemused by what has come with the success of Get Away From Me. Just two years ago, after all, she was singing pop and jazz standards in cabarets and bars in New York. She'd throw in a few of her own songs too and people would chuckle away at the sheer verve and wit of the perky girl at the piano.

Quickly recognised as an outrageous talent, McKay's nights holding up the piano ended when Columbia Records came along in early 2003 and whisked her away from those no-star dives. Enter one-time Beatles and Elvis Costello engineer Geoff Emerick to lend a hand, and exit the anti-Norah Jones with a genre-clashing album under her oxter.

McKay says the swirling styles on the album come from impatience. "If I'm walking around and I'm listening to CDs, I tend to listen to just five songs of a performer's songs or style and then I'll switch CDs," she says. "I guess what I was aiming for with my CD was to take the best bits of my different styles, the best of my influences from Doris Day to The Bangles - or whoever - and to try to duplicate what I liked about each artist and their sound."

It was McKay who insisted on a double album, and she even gave a Powerpoint demonstration to various Sony executives to back up this demand with facts and figures. It's a lovely story, one of many such tales from her colourful back-pages, which may or may not have been enhanced for public consumption. She does boast a suspiciously newsworthy family tree, counting Dylan Thomas, a great-grandfather who was a bullfighter and a grandfather imprisoned on a murder rap amongst its many branches.

Born in London, McKay spent her formative years in Harlem, before zig-zagging from coast to coast with her actress mother Robin Pappas (who now manages her). She didn't particularly enjoy school - "I hated all that assignment work, all that work to keep you busy, I was a terrible essay-writer" - or indeed the Manhattan School of Music where she shored up post-high school, but her return to New York did provide her with a foot in the door of various downtown bars.

Now, though, she remembers those carefree days and nights when the club bathroom doubled as her dressingroom with a touching fondness. At least, there was nothing to stop her writing tunes. "There's just so much now to take care of that keeps you away from writing new damn songs," she exclaims.

She swoons about the idea of "authors going away for half a year to Italy to write their novels", but there doesn't appear to be very much chance of her joining them in their villas in Tuscany in the near future. If all this promotion wasn't enough, there's also a film musical of Katherine Arnoldi's book The Amazing True Story of a Teenage Single Mom that she is mulling over.

"At a certain point, all these things that you thought would be fun before you started to do them just get in the way of you trying to create something different," she says. "You even become unhappy about performing all these songs you've already written and loved. The business zaps your energy. No-one will tell you to stop, so it's down to you yourself to start saying no. I still have the inclination to accept whatever is offered to me, but I'm trying to stop that."

What has not changed despite all of this is McKay's drive and lust for a career. "Ambition is nothing to be ashamed of," she says at one point and it becomes obvious that she sees Get Away From Me as a first step propelling her towards something else. Just what that "something else" is remains to be seen.

"I really want fame," she says, "and that's why I'm prepared to work so hard. The more fame and money you get, the more power you have and there are a lot of things I'd like to change about this world if I could. Celebrity and wealth and fame are some of the biggest weapons for social change, but most people who have them don't use them for anything but getting designer clothes."

If McKay had her way, we'd probably be looking at a new occupant of the White House come January ("I do have fantasies about President Nader") and increased education funding ("Why do they always go on about raising taxes to fund education? Why don't they just spend less on tanks and guns?"). If it's change you want, McKay is prepared to be leading the charge from the front.

"The reason why people like some artists is because those acts are saying or doing something that people would like to do or say, but they don't have the balls or the means to do so," she says. "People are really afraid to put their ass on the line. Just to put your face on a poster and put your name in big print and say 'come see me I'm great,' that takes some cojones. And yeah, I think I have the balls to do it."

Get Away From Me is out now on Columbia