I\'m trying to make (figure out how to..) a 2d platformer in XNA.
I know how to create a tile grid and how to perform collision detection. I perform collision detection
Assuming you don't mind using a third party tool to do the majority of the work for you, you may be better off using something like Box2D for C#.
http://www.box2d.org/ http://code.google.com/p/box2dx/
This library will give you collision detection and physics capabilities.