Welcome to the site mate.
To join a clan is a simple Process First off there are Five clans I will list them and which species they are:
Alpha Draconis- Marine
The Hive- Xenomorph (Alien)
Shadow Serpents- Xenomorph (Alien)
Balatu- Yautja (Predator)
Hunter's Moon- Yautja (Predator)
To join one of these clans look on the main forum and there will be topics to request in, They will be in bold and say "Hunter's Moon" Clan requests, ect. Open up that topic and post "May i join your clan, one of the leaders will respond and say we will watch you which means just be an active posting member and soon enough you should get a response from the leader saying if you're in or not. Each clan has a request topic I just used Hunter's Moon as the example because I'm in that clan.
Hope this helps you out.