Something is preventing me from being active

I feel shy most of times , when Im in class I feel hesitated to ask a question because I feel that I won't get the attention or my questions might be silly.
When I talk with someone rude and aggressive, I get polite and calm because I feel scared of worse consequences, And maybe a fight , so I step back and I try to avoid the conversation even if that person was insulting me ,I don't want to get involved in a problem.
I feel i can't do what I want ,because I might end up in a fight .
What can I do ? How can I overcome fear ? Why do I have these ideas ?