Completely the wrong sub-forum to post this question in, but for basic principles you can check http://en.wikipedia.org/wiki/Reflecting_telescope
. If you actually want to build one from scratch, you have a lot of work ahead of you, starting with fully understanding how they work. I don't know if anyone grinds their own mirrors anymore, so you'd probably start with a finished mirror and build from there, which can be fairly simple or extremely complicated depending on design details, how big the mirror is, and how good a telescope you want to wind up with. Pick up a subscription to Sky and Telescope, you'll learn a lot and get an idea of what you're getting into thinking about building one.