On desktop, press the "Head to Head" button at the top of the page. On mobile, press the "Find Matches" tab.
A list of open and suggested matches is displayed there. You can filter by game and system. If you see a match you want to play, press "Enter." You'll be taken to the match page. Press the "Join Match" button there, and you're ready to go. Talk to your opponent and get playing!
A match that has a "Challenge" button instead of an "Enter" button is a suggested match. Pressing Challenge lets the suggested player know you want to play. If they accept, a match page will be created.
Finally, you can always look for a match in our main chat box. Don't be shy! Let people know what you want a game, you'll find a match in no time.