How do i gain confidence and be more outgoing?

Im always very shy towards people, and i can't stick up for myself in bad situations. I feel bad when people shout things at me in the corridors in school. What can i do to boost confidence and to be more outgoing and enjoy life without having to worry about others. Sometimes i am happy and outgoing but most of the time im scared to be myself in case people dont accept it. I go to a school where lots of things aren't socially accepted. Im rather open minded about things while everyone else is stupid and sectarian.