The first thing to do when starting multiplayer is to choose or create a profile. You don't have the option of porting your offline profile to an online one. No matter how much you've done offline, you'll have to start fresh in multiplayer. After freely customizing your character and choosing a banner, you are given the option to either host or join a game.
To join an existing server, double-click on the match suitable for you and skip directly to the game. Hovering the mouse over an option will allow you to see more information on the server.