Unfortunately, having oily skin does make your more prone to break outs. However, it does mean your skin will age slower than those with non-oily skin, so it's not all bad news.
Oily skin is usually hereditary or linked to hormones, but diet does also play a part. Sometimes it's worth cutting all the junk food out for a couple of months to see just how much of an impact it has. There's very little that can be done about the other two causes. Sometimes birth control helps, sometimes it can make it worse. Acne in particular tends to be linked to hormones and 'that time of the month'. If you find that your skin worsens drastically once a month it may be worth visiting your GP to discuss your options. If you're in your teenage years, chances are things will improve as you get older.
So why do we get oily skin? Over-enthusiastic sebaceous glands. Your skin produces sebum (oil) through your pores to keep your skin hydrated, and people with oily skin simply produce too much oil. Many people make the mistake of over-cleansing, trying to get rid of the excess oil. In actual fact, what this can do is dry out the skin, making your skin think it needs to produce even more oil. Similarly, skipping moisturising can also be counter productive, particularly after cleansing. Chose a moisturiser that is meant for oily skin, and it'll help keep your skin looking matte, without stripping it of moisture.
There are products that you can use within your skin care regime that can help control oily skin. Check out the skin care section for recommended products. And if you're still finding that oily skin is a problem during the day, you can always try using blotting paper, which absorbs any excess oil, without disturbing your makeup.