Summarize the following:
A phone should come up in the bottom-right corner of your screen. If it doesn't, make sure you're not in a cutscene or speaking with anyone. When the phone comes up, press A (Xbox 360), X (PS3) or the Up Arrow key (PC). This should bring you to a menu on the phone. If you have just passed the beginning cutscene, the only option available to you will be multi-player. Otherwise, just scroll down. Click on Multiplayer. Now, you will be presented with a few options. Click Tutorial if you want a tutorial on multi-player matches, click Ranked if you want an aggressive ranked match, which will build up to your rank, or click on Player Match if you want a friendly match, perfect for beginners. If you clicked Ranked or Player Match, you will be presented with the following options...
After the beginning cutscene, or at any point in the game, press up on the directional buttons.