There are many muds on the Internet, as well as many mud-related resources. To start, my Yellow Pages book has an entire section devoted to muds. In order to access a mud, you need a mud client program. The following programs are all good mud clients. If you have never used a mud before, I recommend you start with either Pueblo or Zmud.
Pueblo is simpler than the other programs, and is less confusing to beginners.
Zmud, however, is the most widely used mud client. When you are on a mud and you are having a problem, there will likely be someone around who is familiar with Zmud. All of these programs have features that will seem confusing at first. Don't worry about too many details, and don't let it bother you that you won't understand everything right away. Just get connected to a mud and start to have fun.