Before the Game Begins
When you join a server you will spawn in the lobby area. Enter the ready room once it opens to participate.
When the game starts everyone in the room will be assigned to a team and teleported to the game area.
Afterwards no other players will be able to join.
Start of the Game
Two teams spawn in different locations on the map.
You will spawn without any equipment.
You can identify your team by the colour of their nameplates and your king by his shiny golden crown.
Now its on you to decide what to do.
You can craft a few stone swords and attack early or mine for sturdier materials to get better armor and weapons.
You can build a keep and traps or go for potions, but always keep an eye out for the enemy and protect your king.
What If I Die?
If you were the king of your team the game is over. When playing as a king you should always keep some team mates around to protect you, play defensively and prepare to flee or hide when enemies show up.
If you are a regular player you will be able to respawn at your king after 15 seconds.
A complete list of changes over vanilla Minecraft can be found here.