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...

Summary:
After the beginning cutscene, or at any point in the game, press up on the directional buttons.