On the norwood scale ( google it ) you are a Norwood 2. Fortunately we have effective treatments to help stop and even reverse male pattern baldness. Your best bet is to go see a doctor or dermatologist and ask about finasteride and minoxidil. finasteride a pill you take once every day and minoxidil is a liquid or foam you massage into the affected areas once or twice a day. Do this, and you have a good chance of at least keeping what you got, you may even grow some back too...
Beyond that there are even more effective treatments such as dutasteride, loniten, RU58841, spironolactone and Androcur and many, many more. Get on the first two and hope for the best.. goodluck!
FYI im 20 too and a little worse off than you. Im on loniten and it seems to be working.