first take a calendar and wait for your period. mark an X on every day you have your period. when its over, keep that calendar and do the same with your next period. this will tell you how many days it lasts, if you have an irregular or regular period. and how many days you have between periods. hope this helps you! (:
If I remember right, the site below lets you figure out your cycle and when your fertile time is.
Information on the Rhythm Method of birth control