How effecitive are the prevention pills? If there a chance of me getting pregnant while taking the pill nd having unprotected sex?

Very effective. For women taking birth control pills (ocp's) properly and consistently, the chance of pregnancy is reported to be 0.3% per year, or 1 pregnancy in 333 women each year. The actual pregnancy rate in the "real world" is reported to be at least 3% on ocp's, or 1 pregnancy in 33 women each year. So, yes there is a chance of getting pregnant, but the chance is small.