Zits can be inherited from your parents. This means that if they got pimples at your age you probably will too. In some cases, zits are brought by hormonal changes during puberty.
Under your skin there are glands which make oil to keep your skin healthy. At puberty these glands work harder making more oil which comes up through the tiny holes (pores) in your skin.
Sometimes germs [bacteria] get into the pores, making the oil thicker and harder so that it blocks the pore. This may be black at the top (blackhead), white (white-head) or it can be a sore (red with white pus).