If you are in Assembly Design you can use Analyze --> Sectioning menu to dynamically drag a section through the part. If it is just a part file, insert it into a Product, then you use the above method.
Derek's assembly method is the best, but if you don't want to use a product, here are two alternatives:
1. make a sketch in the section plane, and do an intersection with the part body (do do a GSD intersect between the part body and the section plane)
2. make a sketch in the section plane of a big box that encompasses the entire part. Now do a pocket to remove one side of the part. Make the pocket a different color to highlight the section. (make sure this pocket stays at the bottom of the tree, and you can deactivate/activate it to turn the "sectioning on/off"
