You'll want to click the Multiplayer button in the menu and then get an address for one of the servers in Our Worlds - servers, maps, claims, downloads
Click "Add server" and enter the server address in the "Server Address" box. You can name it whatever you like. Then click "Done".
Frontier (our server running plain, vanilla Minecraft) is still on version 1.5.2, so make sure you've got an instance of that set up in you launcher. In the launcher, click on "New Profile" and, in the profile settings, select "release 1.5.2" in the "Use version" box.
If you're feeling more adventurous and want to try one of the other servers, you'll need to download the Feed the Beast launcher from feed-the-beast.com. It contains several mod packs used on the other available servers. These are heavily modified and a quite different game experience from vanilla Minecraft.
If you are inclined to use TeamSpeak, our TS users generally hang out on the TOG Public TS server. You are certainly by no means required to join TS to play Minecraft with the division, though.