I believe the poster is asking if, practicing NFP for the purposes of avoiding pregnancy, you could only have sex during or immediately after menstruation.
If you are a minor, you really should be asking your parents about this, however, to answer the question in a bit of detail:
During menstruation, you are generally safe to have sex. The same is true for something like two or three days following menstruation. To determine when it is safe to have sex again, you have to tracks a number of different factors, such as body temperature and vaginal mucus. using those signs, you are able to determine roughly when during the month you ovulate. Around three-four days after ovulation, it becomes safe to have sex again until you go through your next menstruation cycle.
If you’re preparing for marriage, you should really take NFP classes. You learn a lot about the woman’s biology, and it helps you both prepare, together, for those periods of the month when you cannot have sex. The length of those periods is generally decided by how determined you are to avoid pregnancy. If you are absolutely determined to avoid it, your only safe windows are those days after ovulation until about two days after your period ends. If you’re only kinda trying to avoid, you could really have sex until around the time you see vaginal mucus increasing. If you’re not trying to avoid pregnancy, then you can have as much sex as you want, whenever you want. It’s something you’l have to decide together with your spouse.