Refactoring And Maintaining Other People's Code

I’ve been feeling the pain of other developers right now. Maintaining and actually changing other people’s code.
I have a couple of thorns in my side.
1. I’ve been using C# for almost two months now. Although I can already say that I a already have above average skill in C# I’ve had difficulty with mainly lack of familiarity of a lot of shorcuts and the best ways of doing things.
2. I am maintaining and at the same time creating a lot of the code. I feel like I am piling mistake upon mistake. I don’t blame the previous programmer, I must confess that I am slowly rewriting a lot of the code, not for dogmatic reasons, purely to make the code cleaner.
3. How Do You Do Unit Test with C#. This is my primary fear right now, If I cant design proper test units, I don’t know how I’ll survive testing this!
I’ll add more as I encounter more difficulty!