Get your hands on a nice , clean, straight piece of hickory or white oak. Those are good woods.
Before getting started, I'd advise doing some reading first. There's a lot of ways to go about it. You have to find what works best for you.
A lot of people suggest red oak boards, but if you lack the experience, you'll just break a lot of boards.
It might cost a little more, but a hickory stave or bow blank would increase your chances of success over a board. If you have trees in your local area, or know a tree trimmer, you could try to get your own that way. Hickory, maple and white oak are good woods.