If you're interested in creating Windows Store apps, then you would use C# 5.0 and .NET 4.5. Note, though, that .NET 4.5 has been updated to .NET 4.5.1 for use with Windows 8.1, and there are a lot of changes.
If you're interested in creating desktop business applications, then you probably already know that businesses often take a while to update to the latest version of anything. This means you could be in situations where you have to target .NET 4.0 or .NET 3.5. This can sometimes be the case with ASP.NET, too, if that's something you want to get into.
I would ask this question on sites where more-experienced programmers hang out. It's not a good fit for Stack Overflow because the answer is partially subjective, but it might be okay to ask this on one of the other Stack Exchange sites.
Also, Microsoft has been hiring a lot of technical evangelists and I've seen some of them posting "office hours." Might be worth looking into.
This post has been edited by T. Graham: 30 September 2013 - 12:25 AM