Find some players who have qualities that you admire (honest players, good boomers, good micro, play the types of games you play, easy to get along with, etc.) and then check out who is in the clan on the website below. Ask a member about joining the clan. Note that many of the largest clans have their own websites; however, some of these have such stringent rules and complex entry procedures that they are probably not worth trying to join.
What you want in a clan is a fairly active membership of decent players, none of whom are known cheaters, stackers, or noob bashers and with a leader who keeps track of members and expels those who didn't abide by the clan rules.
Or, you can create your own clan and invite people to it.