Q:

Which namespace from the .NET Framework should I research?

I am just beginning to play around with .NET (using C#). 
Which namespace, aside from System.*, do I need to research ?
UPDATE: A number of people are asking why research namespaces.
My previous programming experience is mainly non-OOP and VBA (self-taught). 
I have no idea at all to what has already been provided in the .NET framework.
I don't want to sit down and write string-handling functions (for example) to then find out that all the string-handling I could ever need is already written for me (and works better than mine)

A:

Why do you want to start with namespace?
Rather start with windows form applications, web apps, database operations.
While working on these applications, you will need the details of required namespaces and classes.
At that time you can look int only required namespace.

