If you already have experience with Visual Studio, your best bet is to use Mono. Mono is a Visual Studio equivalent for Linux, compatible with Visual Basic and C# code, a great thing for a beginner Ubuntu user that was a .Net programmer on Windows. Eclipse IDE is also a great and very powerful programming tool also compatible with C#. Both are available through the Ubuntu Software Center. If you are wanting a way to develop programs through a graphical interface quickly, your best bet is Quickly, also available through the software center. It uses python however, not C#. But it is very beginner-friendly and is the quickest way to start making apps for Ubuntu. Beyond that, do some searching in the software center and see if there's an alternative that you like better. Try Em all out, if you want. Hope that helps!
Some screenshots:

Mono:

Eclipse:

Quickly:

More information on developing for Ubuntu can be found at developer.ubuntu.com.