Extensive documentation for the .NET Framework can be found at the MSDN web site. Because the URL for specific documentation can change as the MSDN web site is updated, the most dependable way to access the documentation is to go to the main MSDN web site and select the Library tab near the top of the page:
http://msdn.microsoft.com
The documentation includes tutorials, code examples, and a complete description of the contents of the .NET Framework libraries. Spend time getting familiar with the online C# documentation—it's tremendously useful!