When you join the game, you deploy in the main menu where you can shop for new skins and more. When you play the game, you are deployed somewhere with a knife and a random gun on the map so you can start killing people in either free for all or team. Each time you kill, your gun will change. You have to make it to 32 kills so you can get the Golden Knife and your gun will disappear and hit a player with it to end the game. There are many different game modes you can play on, but that is the regular playing style.
You can also unlock stuff like skins and knives but those are just for looks. They come in cases.