As a girl How do you convince people you're a boy and how can you dress and act too seem more masculine?

Attempt to speak at a lower pitch; most girls have a much lower voice than they think they do, and it can be spoken in. If you've got less in the way of breasts, it's easier; wear a tight sports bra/under t-shirt to hold them down and/or learn to move so that they're not obvious. Take hits without showing pain, walk from your shoulders not your hips and wear men's clothes.

Soaper edit: It also helps to talk like a guy as well. Referring to other guys as "dudes" helps. Such as: "Come on dude, let's go" or "Dude, that wreaks". Referring to them as "man" in the same examples works just as well.