how can i prevent pregnacy

The only 100% fool proof way is to not have sex.

Otherwise, using birth control, pills, can be 96% effective or better. Condoms are about 86% effective according to the US FDA. They are often put on late or improperly.

There is also a vasectomy for males and tubal ligation for females. There have been a few reports of pregnancies after such procedures.

Hope it helps.