There's no immediate way to prevent p.E. You just have to learn self-control. Look around the 'net for excercises you can do, and try to become desensitised to visual stimulation. As a last resort, you could try masturbating before sex, but then you may not get an erection at all, it's really hit and miss. There are also sprays available, as well as special condoms to prolong intercourse. Talk to your chemist, or even your gp. In addition, if you're young then you're bound by nature to ejaculate earlier than most people. Most importantly though, don't worry about it! If you worry, you'll make it worse and you'll lose self-confidence.

Masturbating before sex will help you to last longer in most cases.