To play a game online, you have three options:

Joining a Random Game

From the main menu, select quick match. Select the act and mission you want to play, as well as difficulty.

Select 'Search for Games' and you should be allowed to define your search parameters (Same Mission, Same Act or All Acts).

Your console will then search for games that match the same difficulty and parameters. Keep in mind that there are many difficulty modes(Normal, Inferno, Nightmare) and settings (Easy, Normal, Hard) and that only games matching the settings you have set will be available.

Creating a Public Game

When you are in a game, hit start to bring up your network options. You can select between options in this menu to define how your game will be treated in regards to online.

Local creates a local game, where only your console may play. Friends Only allows only friends to join your game. Invite Only allows you to invite others into your game. Public allows others to join your game via the find match feature

Playing with a Mate

You can also simply have a friend set up a game and invite you in. You can have your friend set up a friends-only game if you just want to play together, or set it up as public so other players can join in.

More Advice

• If you are having trouble finding a random game, change the difficulty mode/setting and try again. If all else fails, go for the highest difficulty settings you have unlocked, and look for games in any act. If no games are found, lower the difficulty setting and repeat. If you get to the easiest setting to no avail, lower the difficulty mode, raise the setting back up, wash and repeat. I find there are generally more people playing the easiest settings (players wanting to beat the game quickly), and the hardest settings (players wanting better loot or challenge), not so much in between.

• When you search for a game, getting the error "could not find any games" means there were no games within your search parameters. You will get a "can not connect to battle.net" error message if your issue comes up anywhere before you actually log in to battle.net to check for games.

• Some times it is worth attempting to find games again if it first fails. It is not uncommon to get false negatives when finding games, but very unlikely to get a couple of false negatives in a row.

• If all else fails, you could try networking across forums and forming a group of friends that reliably play.

• Lastly, if you really want to play co-op on a certain difficulty setting, you could open a game and set it to public. Keep near your console and keep an eye out for somebody joining your game. This also sounds a distinct sound effect you could listen out for. Keep in mind that you are not allowed past the lobby of a public game if the host is in a menu.

To do this you have to create a normal game in public, with the person you want to play with online.