Connect your computer to a projector/beamer.
This video is a classic, but many of the techniques discussed are much easier now then it used to be. When this video came out this was much easier then it used to be. When MadMapper comes out it will be much easier then it used to be.
http://www.youtube.com/watch?v=2bRfdn9lNO8
After you are done connecting your computer and watching the video you then need to spend weeks or months watching other videos and practicing. As Mapping is a blanket term for any video that in not confined to a square/rectangle screen it can take on millions of variations. The only real limits in what can be mapped are budget.
I would be remiss not to also mention how much easier a simple rectangle can be or even the preview monitor on your computer then Mapping. You should practice in that 2D realm often as this will give you a foundation in the enormous extensibility of Modul8. Good Luck and welcome to the scene.