Step 1: Get the IP. You can PM me (or anyone who already plays) for it.
Step 2: Install Minecraft if you haven't already.
Step 3: Install MystCraft. Instructions can be found
here.
Step 4: Run Minecraft, log in, click multiplayer, punch in the server IP, and you're good to go.
Important note for those used to single player: You cannot break anything within 25 squares of the spawn point. This is to prevent things like people spawning and immediately falling to their deaths.