Well a clan is basically a group of people that hang together.
Could be on the forums, could be in in real life. either way is fine.
It is best to have some common interest (in this case, gaming of course) and some regular activity that gets everyone involved.
A lot of the GATT clans are based around GLs, Schools and childhood friendships.
For some it can be a real casual affair. A little chit chat here and there.
For others it can be really hard core. Big tournament action. Challenges. T-shirts, the works.
To form a clan, you need 1 leader and 4 members. That's all. Nothing fancy.
Joining a clan is dependent on the rules of that clan.
I belong to [X] and our rule is that you must defeat either the leader, [X]-Neon or the Raiding Team Commander [X]-Baego in a game.
Others will just take you in for a Mountain Dew and pack of Corn Curls. lol
One word of caution! Clan hopping is seriously frowned upon! So don't just join a clan for a week, then attempt to jump out to join another, making the 'rounds.