1. There are three "official" servers. Two of them are usually full with a 10 minute wait time or close to full, while the third usually has 20-30. The actual server that is low will change daily. Sometimes bugged maps, cheating, or other issues will cause a server to lose players for obvious reasons, and it will take some time for it to get back to normal.

2. There are some technical aspects to the gameplay as with Vanilla squad. My recommendation is to try to be a medic if you're new. Medics don't really need to know how to handle any of the emplaced weapons or vehicles, and if you do have a chance to use them, there will likely be people willing to help you learn. As a medic, largely all you're doing is reviving downed team members and healing everyone. Other players can drag players to allow you to revive them from safety (you can do this as well). Even as a medic, you need to have common sense and not run directly into gunfire trying to help someone, or for whatever reason you might decide to push out from a spawn.

Personal note, I never played Squad prior to about two weeks ago. I installed it for this mod and I had no experience at all. I found that asking for help never resulted in someone being rude or ignoring me. As long as you follow their advice or instructions you will likely be fine, and you'll learn more about the game.