Only write what you know is very good advice. I do my best to stick to it. I wrote about gods and dreams and America because I knew about them. And I wrote about what it's like to wander into Faerie because I knew about that. I wrote about living underneath London because I knew about that too. And I put people into the stories because I knew them: the ones with pumpkins for heads, and the serial killers with eyes for teeth, and the little chocolate people filled with raspberry cream making love, and the rest of them.
You've had twenty years of living, and dreaming. You probably have a fair idea of what it's like to experience emotions, and to go places, and to do things, and to change. You've wondered about things you don't know. You've guessed. You've hoped. You've probably lied -- oddly enough, similar skills to those you'll have used in convincing a teacher that you actually did do your homework, but it was stolen by an escaped convict dressed as a nun, will come in useful in writing fiction. Ditto for the skills involved in writing a passing grade essay on something you know absolutely nothing about. Relax. Fake it. Mean it.
And you don't need to figure it all out before you start writing. You can figure it out while you're writing. Or you can fail to figure it out; that's allowed too.
Don't worry about "publishable quality". Just say what you have to say as clearly as you can, and try to enjoy yourself while writing it. Start somewhere, finish somewhere, surprise yourself. And 7000 words honestly isn't really that much writing: a page is about 300 words. If you only write a page a day, you'll have 7000 words in a mere 23 days. And if it really really really really sucks, you'll still have time to write a completely different 7000 words.
(The above owes not a little to Ursula K LeGuin, who I would have quoted directly except that as I write this the sodding journal isn't loading properly and neither are the archives with her quote in -- but it's somewhere in January 2003. The observation that if you just write 300 words a day you have a novel in a year was Stephen King's.)