Random Match will be an option under the GTS. Here, you can go in and select the various Random Match styles you want. You'll be able to play with people over WiFi who aren't in your friends list.
Rules
Pokémon of Level 51 or above are lowered to Level 50 while those under Level 50 will remain their current level
You cannot have two of the same Pokémon or item in your team
Items that aren't available in Generation V as of yet, including event Berries, cannot be used
No Pokémon with the move Sky Drop can be used
Two minutes to decide upon the moves to use
Matches last no more than one hour
Source
