Learn your personal cycle
If you are trying not to get pregnant you should always use protect. Even if you know your cycle it is always possible to have it be irregular one month and you may be able to get pregnant when you think we can't. I suggest always using some form of birth control because you could get pregnant at any time.
Now if you are trying to get pregnant I recommend that you track your cycle. The first day of your period is Day 1 of your cycle. On average a women's cycle is 28 days long. Many women will have longer, shorter, or even irregular cycles. Tracking your cycle will help you to be able to estimate when you are ovulating. In general you will ovulate on Day 14 or between Day 11-16. Your egg will be viable for 24 hours after ovulation so make sure you have sex if you are trying to get pregnant.
If you are trying not to get pregnant use caution at all times!