3 pieces of advice:
1 Don't look for a girlfriend. Treat guys and girls the same and just socialize with everyone around you and be open to activities you normally wouldn't do. Being socially active will get you invited to parties, concerts, clubs, whatever as part of a group. This makes meeting new people much easier since you have friends to fall back on and help you relax if you get tense, and you will inevitably be introduced to girls in a context that makes conversation very easy. For example you and a few buddies go to, say, a curling match (or whatever, something new to you) and someone brings his girlfriend. That girlfriend brings a friend, the friend brigs her sister. You now have 3 girls to interact with - the girlfriend, safe and easy since you shouldn't feel the pressure of trying to impress her (you shouldn't really try to impress anyone, people should like you for you, you can't hide who you are for long anyway), good for building up your confidence; and the friend/sister. Just discuss the activity you are involved in to break the ice ( just tell them you've never been to a curling match and your opinions on what you've seen so far. Probably best to be honest about it, just don' go calling it plain crap. Have some arguments ready). If you really can't seem to strike up a conversation, don't force it too much. It will bore them, and make you more uncomfortable. Enjoy the game since there's always next time.
2 First impressions are VERY important. While it may seem vain or shallow, people ALWAYS judge your appearance first and then they decide if they want to get to know you or not. You might be truly charming, intelligent and funny, great company and whatever else you like, it's all for nought if people won't even return your helloes because you look like a hobo (exaggeration for comedic effect). So good grooming and a bit of care choosing your outfit are very important. Try to get in shape, get some new clothes, get a cool new haircut. Again, vain though it may be, looking good WILL make it easier for you.
3 Don't skulk in the back of the classroom, and don't try to butt in every conversation. Get noticed without trying to be the focus of attention (I'm assuming here you are usually not the life of the party, if you are, then disregard the previous). Try to be honest about who you are and what you like. You are a person, with opinions, likes and dislikes, don't try to change that just to please others. In the long run, being honest might land in you fewer relationships(friend, work, or more), but they will be better quality ones.
Also be very open to new stuff (music, activities, sports, anything). That will get you in even more social situations with new people and you never know who you will meet or what you might end up enjoying.