You ARE cute--it's just hiding in there, and you need to work a little bit to bring it out.
Go to JCPenney and get color-typed, they have this service at the makeup counters to help you find the colors that complement your skin tones best in makeup and clothes.
Review your wardrobe carefully, and THROW OUT anything that you don't look cute in. Go and shop anywhere, everywhere, including secondhand stores, to find the things you DO look cute in, that are in your colors and cut to flatter YOUR figure. Try more solid colors than patterns, when you find your good colors. Guys like solid colors so that they can see your shape better.
Get some bath salts and skin softeners and work on every inch of your body, smoothing with pumice stone and nail file kit. Make your feet the cutest feet, your hands the cutest hands, etc. until ALL of you is just TOO CUTE!
Find a fragrance that enhances your natural pheremones (again, JCPenney is a good place, because they have tester bottles out on the cologne and perfume counters for you to try on) and mix it with something regular, like Jean Nate or a nice-smelling deodorant so that it isn't easily recognizable. Then you can smell cute.
Get your teeth whitened, and pay special attention to your hair. Go to a stylist (at least one time) to get a good idea of the best cut to flatter your face, and then get that cut if it doesn't require too much maintenance. You need a hairdo that you can get all touseled and still look CUTE.
This is what it takes to be cute. Pretty girls are ugly if they don't do these things and work at being cute, and ordinary girls are cute if they follow the rules listed above.