The big trend in playing D&D online is a free program called Maptool. It's a combination text-based chat and map program that can be as simple as a visual aid for your encounter maps, but it's also a powerful programming tool and there's a large community of coders on the site's forum.
Some groups prefer to have voice chat as well, to enable things to move faster. Mostly that comes down to Teamspeak or Vent, though I've seen at least one group require Skype.
There are places to find a group on the various D&D message boards. RPTools (the Maptool site) has its own board, which is naturally going to be populated by folks who're a little more involved in Maptool's features. WotC has a very brisk forum, it's also mostly populated by younger gamers. EN World is traditionally more of a GM haunt and so you get older folks there, and many of the classifieds are strictly for tabletop meetups. Another venue is the RPG.net forums, I can't offer any insight there since I've only rarely been.