As earlier mentioned you can Google it, or ask your OB about it. The main idea of the method is to make full use of your non-ovulating days by logically plotting an estimate BASED on your monthly cycle. So may computations yan, may mga changes din yan if your cycle is irregular, so your OB is in the best position to help you plot it especially if you are clueless about it. The more you do not know anything about it, the riskier it is.
Is it reliable? Reliable enough but if and only if your cycle is regular and continues to be regular and if you practice it properly.
Is it safe?
Among all the birth control methods this is the one with the lowest rating. Not even 90% effective when practiced by the typical couple, and not even 50% effective when practiced by unwed lovers who are just having a fun time banging each other regularly.
Still, I say you should consult your doc about it. I'm sure she'll be able to convince you to use some other form of birth control.