Need Assistance in Building a Gaming Desktop Computer

I recently decided that my Sony VAIO laptop is a little bit on the bad side when it comes to gaming. I'm looking to build my first gaming desktop computer (have built friends computers)that would be under my budget of around $650 CAD, would be able to run most new games at high detail with smooth frame rate (for example, a game like Skyrim would be at the top of the chart) and would also be able to run AutoCAD 2013 with no slow response in the program itself. I've never bought parts online before so any help on that would be greatly appreciated. Thanks in advance :)