Here are a couple of things you should know before you do:
If you'd like to use FA as birth control, here are some very important things to know before you get started.
Using FA for birth control requires knowledge, discipline, and complete adherence 100% of the time. If for any reason you're unable to do any of those things, please consider using a different form of birth control. If used perfectly, Fertility Awareness will work perfectly.
Find a certified Fertility Awareness instructor to help you understand the rules of FA. You can find a teacher here.
In addition, I highly recommend reading the book by Toni Weschler and by Katie Singer. They will serve as great reference guides for you, and will go into a lot more depth on topics I can't fully cover here.