Getting started on your own recipes is as simple as diving in and experimenting. I started off by researching what other people had done for their beers, and I would do something close with a little bit of a difference to make it my own.
What really helped me was a book called "designing great beers". It's a little lengthy and maybe not for a new brewer, but it's the best resource out there for recipe creation...in my opinion. It mostly helps the all-grain brewer, but if you're doing extract, I'm sure you'll find some benefit from it too.