NFP involves using various methods to determine what stage of your fertility cycle you are in. There are different methods taught by different groups.
The Couple to Couple League teaches the Sympto-thermal method which uses temperature, mucus, and cervix readings cross checked to determine your cycle.
The procedures sound difficult but that is due to the fact that the books have to cover ALL variations in fertility cycles, like post-pregnancy, coming off the pill, approaching menopause, etc. Once you learn the procedures, it becomes pretty easy.
As was mentioned, check for a course in your parish or diocese. Check the following web site for teachers in your area:
ccli.org
All the teachers in Couple to Couple league are volunteers so you only pay for the supplies and a membership in the Couple to Couple league. If there is a need, scholarships are available.
It is good that you are looking into the pre-marriage. We decided to look in it after the birth of our first child and we wish we learned it earlier. We knew we wanted a child after marriage so we didn’t care about finding out about NFP yet. But it is good to have a few “normal” cycles charted before you get pregnant.