As Zeno said, first step is to pick a codebase. I'd suggest something you're familiar with, what type of muds have you played in the past?
What you need to compile it varies from base to base, but since most muds compile on some form of *nix, you'll likely need Cygwin to use it on windows. I'd recommend getting a host, there are several reasonable ones available and it's generally not too difficult to find some space on a linux box. This should be good enough for early development.
That's all you really "need" to get started with a mud. Most muds are written in C rather than C++, although some can be easily converted. To familiarise yourself with the code, I'd probably start by changing the messages around the mud. You might want to hold off on updating commands like score and who, since they tend to get complex.
I'd pay special attention to the builder capabilities of the game. It's often easier to use or modify existing features to get what you want, and players experience the game through it's areas.
That's all I've got for now. If you have any more questions, let us know.