Things you will need:
A .NET program, prefferably one you dont know the source code to.
Download reflector, and run it. Reflector is the one labeled reflector, it has 3 gears one it as an icon.
Click file, then open on the top left. Select the .NET executable, or even dll.
It will popup in the list below labeled the same thing as what you selected. If it is red with an exclamation point, its not a .net program, so it wont work. If its not red, then good job its a .net program.
right click that and click disassemble. Now click the + sign and go through the source!
You can change the language that it displays in, like C# or VB, by clicking hte dropbox and selecting the language
If its unreadable, then its obfuscated, decompiling an obfuscated program requires more steps =/