Since you're using Stata 12 all of the user manuals (and help files) are already installed. See -help- and try the Help menu -> PDF Documentation. I would suggest reading the Getting Started manual and the User's Guide - doing this will be well worth your time in the long run. There are also stacks of online tutorials, with the most comprehensive collection coming from UCLA.
Your graphs are overwriting one another because you haven't named them. You can name them using the name() option for twoway graphs. Here's an example: