I have a laptop in my hostel room with me which I use only for entertainment purposes. But now I want to use it constructively, I want to maximize its potential.
Can someone tell me how to do so?
(by constructively I mean part time jobs via internet, enhancing my knowledge etc... )

You can make money online, but it's easy to be scammed or to make costly mistakes.
Sites such as eBay require that you be 18, and of course you need a Paypal account with a bank and credit card attached.
Some people sign up for information and how-to sites such as ehow, which pay a few pennies for each hit. It isn't easy to get in, however, and I don't know if they check age.
I hear that google is starting to pay for information givers.
Other people make money by allowing advertisers to appear on their page when a video is watched, for example. Google AdSense might be one way.
You can actually sell on Facebook, starting with friends and expanding through their shares, or through key word search.

Enhancing your knowledge is easy - most of us use our computers for that daily. You can just type in some words (human evolution, e.g.) and start reading and read for months. I've done that and even taken notes, as though I were in school. Or you can find an online degree program like Phoenix Online. MIT lets you take their online courses for free, but you get no credits.