How much C++ do you know? Basic vs advance varies from person to person and their understanding of the language. Are you familiar with inheritance and templates? Do you know how to construct a graph?
A typical mud is a large graph, where each node represents an area and its edges dictate what direction you can move in. Each node would contain, for example, a modifiable description, a dynamic list of people, monsters, items, things that can be interacted with, etc. Brainstorm and write down all the nouns in your game. Use a UML editor and figure out the relationships between classes. Use inheritance where logic is common amongst multiple entities, or use templates if you want to break the entities down by type. Think about the NPC engine and how you will use "smarts" to move NPC and monsters around. Maybe define a zone where they will dwell for example.
There's a lot more game mechanics you will run into, such as AI, turn based combat, large scale graph handling, serializing, etc. These are topics you should think about as you architect your game. I don't know of any MUD dev how-to websites, but just think about it. It's not terribly difficult to conjure something up.