Acne,and, well, spots in general, are caused by colonisation of the skin by one type of bacteria. usuallyyour skin has all differenty types and they keep each other in check.
When the Ph of the skin changes (usually in response to hormones, ie when you start puberty, or for a lot of girls, at certain points of menstual cyle) it kills of some bacteria and allows others to colonise, which in turn leads to infected sebacious glands, inflamation and spots. The best thing to do it keep you skin at a neutral ph (7, like water).
as others have said, plenty of water is good, as are Zinc and vitamin C supplenents (they will also help improve you scar tissue). But the key thing is to use GENTLE clensing products to clean your face, and then to moisturise afterwards. Often people with acne use harsh products (like clearasil) which strip away all the oils, which put skin into overdrive meaning it makes more oil and starts a vicious cycle.
consistnacy is the key to skincare so use the same product (something like simple or johnson and johnson or a supermarket own prand product (tend to have less chemicals in which are likely to irritate your skin)) twice a day, keeping the area constant. dont switch and change, it will cause fluctuations and further colonisation. Also moisturise, because as i said, if the skin gets dry it will make more oil.

hope that helps!