Once you're level 150, you can apply to join a clan with the proper gold and room inside a clan. Each clan can have up to two leaders - Primary Leader (who created the clan usually) and Co-Leader that can be swapped out at any time by the Leader.
If you want to start your own clan, head to 2,2 Royal University with 20M gold and at least level 300 to do so. Adding clan members cost 1M * #clanmembers. For example, if someone wants to join and you already have 5 clan members - it will cost 5M to invite someone else.
Each clan has a designated place to chat - in the Race/Clan Chat section of the game. Other clans cannot see your messages. Leaders in chat are designated by LEADER and CO-LEADER titles automatically applied to their name.
The Leader of the clan has absolute power - Can set a Co-Leader, accept new players, kick players from the clan, set the clan bonus, and interact with the clan hub in Xandor Kingdoms once built. If the leader leaves the clan with no co-leader, the clan is disbanded - otherwise, the Co-Leader becomes the leader.
The Co-Leader has the same power as the Leader, so choose someone you trust! If the Co-leader leaves the clan, the co-leader just becomes an empty spot to fill.