From the mainmenu, click on 'Multiplayer'. A new screen will appear with the heading 'Arena Servers'. Below the heading there will be an option that says 'Servers:'. Click on it until it says "Servers: Internet". When "Servers: Internet" is showing, a list of internet servers will appear below. Click on one to select it and then hit the big red fight button on the lower right and frag away.
This is much harder to explain than do.
Play around, you break anything.