i'm suffering from alopecia areata now and i've been constantly losing eyebrows

Alopecia areata can happen in areas of the body other than the scalp though the scalp is most common.  In many cases, the hair will grow back within a year so this is encouraging.  A lot of the times it is hereditary.  There are some treatments available, but I am not sure if all of them would be acceptable for use in teens, for instance, the steroid injections.  The creams and shampoos they have available like Clobex or Lidex may be used since it goes onto the scalp itself.  I hope it all gets better for you soon.  In the meantime, I know that many guys shave their head completely as it is the “in” look. If you think you can pull it off, go for it! ;)

I want to know what are the causes of alopecia/baldness and what medications can we use?

nothing to be done I'm afraid and if your eyebrows are touched it seems pretty bad to me because it's not only alopecia areata which centers on the top of the head but it's evoloving towards alopecia universalis which extends itself to the whole body. your post is 20 days old, it may even go very fast. where are you at? what about armpits and pubes? I had it when I was turning 17 it came out of the blue, nothing hereditary (I mean both my parents have bad hair, my father was an early bald and my mum has androgenic alopecia but this is something completely different and unrelated, on the contrary my mum has much body hair), and it took 6 weeks and I had lost every hair I got everywhere. I used to be very hairy, like my mum and the last hairs to go were the ones on my breasts. So be prepared but you'll learn to live with it and you'll be beautiful despite of it.