First of all, it depends on the menstrual cycle length, and that often varies in early years of reproductive life. If you know that your cycle is regular over a relatively long period of time, you can calculate when the ovulation (release of ovum from ovary) is occurring. All bets are off the table if the cycle is irregular, as you see, the prediction is based on past experience, and if past experience tells that cycles are irregular, you have no way to determine with some certainty as to when ovulation is occuring. In general, a normal cycle can be as short as 21 days and as long as 35 days. But, then there could be abnormal cycle length.
The menstrual cycle can be divided into two parts, one before ovulation and one after ovulation. The length of the part after ovulation is relatively fixed (14 days). So, for example, if you have a regular 30 day cycle (from beginning of bleeding in one cycle to the day before the first day of next bleeding), then you can predict that your ovulation is occurring on day 16th. Since a ovum can survive an average 24-48hours, by not having sex between day 13 to day 19, you may be able to prevent pregnancy.
However, one should remember that no such safe period can be determined with 100% accuracy. For example, the cycle length may increase or decrease for stress and anxiety (which is likely after marriage) and other reasons. It is advisable that the husband either ejaculates outside vagina (Ajal) or use a condom.
Hope this helps. And, congratulations on your marriage. Wish you a happy conjugal life.