Think of clans as sporting teams of sorts. People join clans for a variety of reasons. They generally participate in organised wars against other clans which may or may not be part of an organised compettion among many clans.
Don't be in a hurry to join one. You can still play and get all the enjoyment you want without being in one.
If you regularly play on one particular server which is run or frequented by a particular clan/clans they may (after some time) ask you to join if you and your style of play suits their clan type.
I am part of a mature aged (over 30's) clan in Australia. We own our own server and run it the way we want. We don't tolerate the "kiddies", hackers or just plain tools at all. We also run custom maps. We pay for the maintenace, upkeep, upgrade and running of our server and that is what our clan is about. We aren't in any organised competitions but we do war occasioanlly with other Day of Defeat:Source clans for fun.
There are many many clans in the many online games, if you want to be part of one, I suggest you hang and play with players you like to play with and they will probably invite you aboard eventually.
Can anyone else explain it better. I think I gibbered on a bit.