Looking for co-author/writing buddy for YA contemporary romance/mystery

ALEXA, you're asking a great deal, which is likely to get you very little interest. People are rarely eager to write or co-write someone else's idea for no payment, or the promise of a payment when and if the work sells. Six-figure advances are imaginary.

Maryn, hoping you stick around and get fired up to write it yourself

Welcome, Alexa!

As Maryn noted, most writers have a zillion more ideas than they ever have time to write them. Learning the craft of writing is the hard part -- conventional wisdom says it takes ten years and/or a million words of writing to 'get there'. And once someone has got good at it they've likely got a shedload of story ideas to choose from.

Sometimes co-writers do team up, but they're usually at the same stage in their career. New writers might do it for fun and to learn from each other; established writers do it to cross genres or boost sales. But, nearly always, co-writers have first established a strong relationship before they contract to write together.

And, of course, most authors are lucky if they get a $5000 - $10,000 advance for their debut novels. So maybe your aspirations are a bit unrealistic!

I'd suggest hanging around Absolute Write, and getting involved in the critiquing workshops here (in Share Your Work). Writing critiques is hands-down the best way to improve your own writing, and once you've participated more around here and people get to know you, there's a better chance that people will put their hand up to work with you and help you out. Give it a try!