U can calculate the days of ur cycle when u r most likely to get pregnant as follows:
How many days ar there usually from the 1st day of ur period to the 1st day of ur next period? Usually, this is 28. Divide this duration of ur cycle by 2 (28/2= 14). Dat means, in the case, the 14th day of ur cycle (counted as 14 days from the 1st day of ur period) is the day u ovulate, ie-the day ur egg leaves ur ovary.
U can get pregnant on this ovulation (14th) day + upto 4 days before this day, as the sperm can survive in ur body and fertilize an egg ovulated, upto about 4 days. Also upto 3 or 4 days after the ovulation (14th) day u can get pregnant because the ovulated egg can survive in ur body for about 3-4 days.
So, in summary, if u have a 28 day cycle, u have the highest chances of getting pregnant (ur fertile period) is from the 10th to 17th-18th day, counting from the 1st day u get ur period.
(For another eg, if u have a 21 day cycle, ur fertile period is 7th to 14th-15th day)
However, remember that there are cases of getting pregnant on other days of the cycle as well, during the so-called 'infertile phase' of the cycle, so best use birth control. (While on ur period, u r generally considered to be in this infertile phase)
Hope this was of help:-)