Everyone can use magic. You just need to learn how. Belief is a big part of magic but it's just one part of it. You also need to understand magic: what it is and how it actually works. Learning meditation, visualization, energy flow/control, and so on can also really help you successfully cast spells. If your spell is something to turn you into something else of give you wings or anything else along those lines, it wont work either.
I added links below to some areas on the site than could really help you out.