Look up SmartFox Server, they have software to set up a server and plenty of documentation. I’ve never used it, personally, but it’s very popular and I assume there’s a good reason for that.
I’m working on a realtime MP game, and I’ve been using MMOcha (mmocha.net), and it’s got some nice ping times. That’s made specifically for realtime stuff (IE StickArena etc), so if you’re not needing the extra speed, I’d probably go with SmartFox just because of all the extra documentation. It would probably help you grasp everything a little quicker.