Er, ignore the one telling you to 'get a life or play sports' – I don't know what that guy is on.
Tea tree oil is great. Trader Joe's, if you have one near you, carries a face wash that is superb for preventing and treating acne. I inherited my mother's combination skin and it helps immensely with stopping acne breakouts or blemishes.
Diet can help, but it's important to know what aspect(s) of your diet may be the major cause. The skin is an organ like all the others. Skin problems may just mean that your body is trying to get rid of waste material it has no other way to dispose of, so it is definitely advisable to look at what you're putting in your body and maybe think about improving your diet.
Try to avoid touching your face too much. Fingers are terrible about picking up oil, bacteria, and various other substances you don't want on your face. Wash your hands regularly.
Avoid applying oily lotions to your face, in general. If you wear any kind of makeup, be sure to take it off as soon as possible when you're done with it. Never sleep in makeup. Some people can also be sensitive to using things like hair spray, which can clog pores on the face, so you might want to wash your hair and face after you're done with your style for the day.
Keep your face away from oil steam. If you're frying anything and there's oil and steam, it can be really bad for your complexion. Deep fryers are the worst for this, since they atomise the oil and other substances, which then become airborne. There's a reason why people tend to smell like fryers if they spend any time at all around them: it gets all over their skin and the fabric of their clothes. If you are around something like this, wash as soon as possible and change out of those clothes. You don't want that against your skin for any length of time if you can avoid it.
Try to keep your stress levels manageable. Stress is devastating for health, and that means that people more prone to acne breakouts will be more likely to develop them in situations of long-term high stress. There are also a lot of other things made worse or caused by persistent high stress, so please take care of yourself!
Basically, prevention is far better than treating the symptom, which is the acne. There are plenty of over-the-counter creams, ointments, and other solutions. I recommend washing your face regularly, though of course don't overwash it. If you just change a few things and mind a few other things, you should see some improvement with just a little easy here and there damage control.
Hope this helps!