Why won't my 3G work on my iPhone 4S?

I have a iPhone 4S and my 3G won't work I have 1gb of Internet but it won't work? At school or outside? Most of the time ill have 5 bars of signal but it still don't work! POINTS FOR BEST AWNSER.

That is because the sim card you use does not support 3G to your area

Bra that sucks. I just need points so disregard this answer.