If you want to improve your appearance, you can to a certain extent. Dress well, in a way that suits you. Have a look in Topman/ H&M. Get a decent hair cut. Shave any unattractive facial hair and be well groomed. Get some decent deodrant. Treat any skin problems. Exercise. Write yourself out a healthy diet plan and stick to it. Don't starve yourself, make sure there is plenty of healthy foods: fruit, veg, brown bread. You can still eat junk, but keep it to a minimum, for example, a flapjack once a day.
Go out with a positive attitude. People will want to be around you if you are comfortble with yourself. Don't let looks hold you back, because as I say there is only a certain amount you can do about it. Have you seen Ugly Betty? She is cosmetically-challenged, but people love her.