Giving credit

What would I do if someone used something you created without giving you credit?

To be perfectly honest about it I am not so sure what I would do. I seem to have the reputation by everyone that I know that I am why to nice and I seem to let a lot people get away with things that I should not. If I found out someone had stole work of mine and passed it off as there own, I would be really upset but I probably would not say anything. Its not because I don't think it was wrong but mainly because that is how I am, I tend to keep things bottled up inside and and never speak up especially when I should. Now to get right down to it if both of us was pulled aside by the teacher or who ever and was told that we had identically work then that is when I would speak up about it. I am trying to work on the fact that I should speak up for things that are not right.