I'm really not trying to be ugly, but a gym membership would be a better investment than buying stylish clothes. You don't have to be ripped with six pack abs. Exercise and a healthy diet will help you drop a few pounds. You'll have more energy and more confidence. Girls respond to the energy and confidence not so much stylish clothing.
Moreover, dropping a few pound will force you to buy new clothes; your old clothes will be too big. You'll look a lot better in your "thin clothes." Best of Luck
Buy clothes that fit you well and that you feel comfortable in. A cute girl is cute girl if she's in a prom dress or t-shirt. The same applies to men. If you haven't developed you own taste or style, denim jeans and a plain t-shirt or a button down tee is a pretty safe choice.
OK I might catch a lot of flack on this but I'll say it anyway because I don't, in general, give a sh*t about what people think about me: Loose the frickin' weight! You need to hear it! It'll do you good, trust me! Think about being overweight as a disease! Because, guess what, it is! Put a comment in here if you want to know how! I can help you.
Bottom line is, if you have a reasonably good body, you don't have to worry too much about what to wear because they will make you look good regardless. Remember Al Bundy's quote: "Honey, it is not the dress that makes you look fat, it is the fat that makes you look fat!"