RAT is actually written in VB.NET, I find it easy and pretty quick to do what I want to do. Most people will tell you C# if you are going to learn a .NET language and I can't disagree with that. But... I like quick and easy.
Really though it depends on what type of applications you want to write. If you want to write Windows desktop apps, then any .NET language (for the most part) is good. If you want to write web applications then... you can do it with .NET but I think there are probably better languages out there. Just so many to choose from... Really though, learn one language, then the others are easier to learn. I know probably a dozen or more language well enough to write code in them, and about 3-4 really well. If you have other questions though, let me know. I love coding and do it for a living. =)
Ohh also, nothing is beyond you to learn if you really want to learn it. =) The key I think is having a project you are passionate about to keep you interested through those times when all you wanna do is smash your keyboard (those will happen, a lot. )
Depends, but typically for RAT I use Visual Studio Community Edition (its free). Other than that, many tools for various things like Navicat for SQLite database editing, notepad++ for various notes and such, FTP apps, etc. But mostly for code, its all Visual Studio.