When you join the clan you will be by default placed into Green Team, Green Team is the squad that handles the main infantry position. They run head first into battle and hold ground and repel the enemy They are in control of explosive weapons and other strong weapons like spartan lasers, rockets, fuel rods, ext. So if you like using explosive weapons and the other guns i mentioned you;ll want to stick in green team.
After you play with the clan a little you will notice the two other squads, Red and Blue. Red squad is the precision weapons squad. They deal with sniper rifles of all kinds and long range marksmen weapons like the DMR. They also do a lot of stealth work and play a large defensive role. Blue squad is the vehicle/CQC squad. They Have control of the tanks, warthogs, mantis, and banshees. They also are the team that infiltrates the enemy base to capture the enemy flag and bring it home. Red and Blue squad aren't as easy to enter as Green because you have to ask the squad leader and they will most of the time request a sort of testing. Each squad as priority or "dibs" on there squad weapons. If a green wants to snipe but a red is present the sniper goes to red. Same with vehicles if there is tank by you and a blue is coming for it its theirs. So basically if you like sniping join red, If you like driving vehicles join blue, and if you like just running around killing join green. You can also switch your squad any time with no penalty.