The type of birth control that SK talks about is a 3 monthly injection called Depo Provera.
https://www.rwh.org.au/womensinfo/factsheets.cfm?doc_id=6135
The more common type of birth control comes in the form of the pill which means you can only skip about thee months worth of bleeding before your body will choose for you to bleed anyway.
Doctors are often reluctant to give the pill out to pre-teens because it is deliberately messing with your hormones and basically long term use can produce fertility problems later in life. Doctors are even more reluctant to give the injection unless special circumstances exist such severe heavy bleeding or cramps. Also, remember that the effect of each type of birth control is different for each person.
For example, on one type of pill I can skip my period for three months... on another one I tried recently I would bleed randomly regardless of what section of the pill I was on! You have to find one that is right for you.
More info on the pill:
https://www.fpahealth.org.au/sex-matters/factsheets/54.html
Also it is a good idea to maybe let your body bleed naturally at least for a year or so until it works out what it is doing. In the beginning your bleeds will be irregular and a bit all over the shop. It is always a good idea to know what is normal for you which you will only know after you periods have settled down. That way in the years to come if you have to come off for a while for any reason, you will know if your body has readjusted to its natural course and you will be able to detect bleeding abnormalities which can be indicative of cancer.
Don't be in too much of a rush to get on to the drugs!
Note: As far as sex goes... the pill only protects you from getting pregnant. It does not protect you from STDs so if you are on the pill make sure you use a condom as well. STDs are generally for life!!!! Until you are engaged or married, you don't know if you will be with someone forever, always play it safe... for you and your partner!