Pimples form from oils, and dirt if your face. The more you touch your face, you put the oils on your face from your hands and dirt collect on your face creating a pimple.
Ways to prevent your self from getting pimples:
1) Was your face daily, the more you wash your face they cleaner it will be. Remember only wash your face twice a day.
2) Try not touching your face as much, letting the oils on your hand to meet with your face will make a pimple. Try avoiding touching your face.