As you join any game, you can click any block to open options.
Here you can see some options (some of which may not be available for some games): Teams, Kits and Hotbar Editor.
In Teams menu you can set the team you want to be in. But this requires having a donator rank.
In Kits menu you select one of available kits in the menu. First kit is always open. You can get all other kits by claiming XP rewards. After selecting kit for the game, you don't have to reselect it after each match.
Hotbar editor allows you to move items in the hotbar as game starts with your kit. For example, if kit provides sword on slot 1, then if you have swapped it with slot 7 in hotbar editor, sword will appear in slot 7. Each game has own hotbar settings.
You can see we have an XP system. To see how much XP is needed for new level, or how much do you have, look at sidebar. Each game has own XP.
You can vote for your desired map or pvp mode. Voting is allowed until 5th second before game starts. Then vote system selects first most voted map in the list (first most voted means if 2 maps have same vote amount, first one will be selected) and most requested PVP (if there are 1.8 players on the server 1.8 will be automatically set, but if Donator votes for 1.9, and 1.9 wins the 1.8 will spectate and game will be 1.9).
After counter is done, some games provide 25 seconds to look around map.
Then the game starts! When game started, there is one more thing: in some games you can use In-game Shop to buy items on ingame coins you get when achieving some goal (may be killing players).
You can Join a match once it has already started and spectate when you just want to watch.