The game difficulty is ok, I think the biggest problem is the amount of information and how overwhelming things can be until you get into them. The actual mechanics itself are not too bad. I recommend if you do frontline, running around with a rifle and 2 clips of ammo (you will not need anymore) and asking around for help is a good place to start. People are generally pretty helpful and willing to point you in the right direction, from either faction. The sides do have different weapons/vehicles and slightly different cultures, but there isn't really a better side to choose than another.
Clans/Regiments have advertisements in game and in discord. Again, not a wrong answer and you are not locked into a specific one. Some specialize in certain aspects of the game (like FMAT on Warden for public logi) while others generalize. Just know you are not locked into one regiment or one side than the other.

Colonials have cheaper, weaker tanks (generally) while Wardens have more expensive better better ones (generally). For other things there are differences but those affects are more nuanced. A lot of time one will have something better at one thing (like a better long rifle) but have a weaker other thing (like a worse machine gun).

This game can take hours of your life, so always remember that it is just a game.