Boundaries....

Where does boundaries start? and how to keep them once they've been set? I don't have a lot of boundaries...I like to think I consider others when making my decisions and how that decision will effect them. I think setting boundaries is like making the statement "it's all about me and this is what I exspect". I know I need to do this sometimes...I just don't do it all the time. I set boundaries with my co-workers. There are some I don't want to be around but I'm a team player...I just wouldn't choose them for a best friend. I really don't think I have a really close friend at work but I do have one I don't mind sharing some personal stuff with. I guess that's good. I guess I really don't have a clear understanding of what I should and shouldn't set as a boundary. That leaves me open to be used. I don't think people do it on purpose...well some do...but I guess I'm a giver and I just don't know when to stop and say enough is enough. Therefore I'm mad at myself most of time when I don't convey a boundary that should of be set. I should be a stronger person then what I am. I just want to be accepted and I fear rejection. There it is....that's my boundary....fear of rejection. I'll do anything as long as you don't reject me. Now's the part where I have to get over that fear...but how?