screen coord

I'm trying figure out how go about doing a crosshar for my game. My game is 2d side scroller. I got the cross hair working easy enough, but I'm confused about the whole camera.worldtoscreenpoint and ray casting. I don't understand it or rather a method to apply it. if any one knows a tutorial on subject or book please lead me in right direction. I not asking for hand me out. I'll put time in learning it and I've googled it about three hours now, but I just silly questions on Unity answers.....Kind like my silly question haha.