Before you start the game, you need to set the game mode to death match and not free for all. From here you can choose how many teams you want.
If you get the same player on your team every time, then this seems to be a bug in the game. If this happens you should set your teams before you start the game and then when you go the start the game leave the teams to custom instead of changing in to two.
Depending on how many teams you want you need to make sure that there are enough spawn pads for the teams. If not, you’ll find yourself spawning in the sky.