One of the common side effects of b/c is weight gain... it happened to me - however it doesn't happen to everybody. Weight gain is a result of an increase in hormones (from the birth control).
Birth control can also be used for other reasons and can have other side effects, such as heavier/lighter periods, increase/decrease in acne, nausea, headaches, dizziness... the list goes on.
Talk to a doctor (they must remain confidential and cannot tell your parents) about the birth control - they will be able to let you know what to expect.
Well...I have no idea on the subject honestly, but I think we'd see a lot of girls that aren't sexually active taking birth control if this was the case.
Although from a simple google search it seems this is common knowledge...huh...
http://www.enotalone.com/forum/showthread.php?t=172111