Be yourself.

You can work on your body, try to act like a retard/cool boy, change your clothes and what ?

You'll meet girls that don't suit you.

Don't you prefer a nice shy girl than a sluttish big mouth ?
And more, what to do after ? You'll lie the rest of your life just to touch girls once in a month ?

No, your real problem is that you are not confident with what you look like now. Advice : find a little job this summer. This way you'll see what you can do, get some muscle, have contact with reality and win money.