Sometimes missions are posted and anyone can join them, but most time its a free for all. If you look at the map , the large concitrations of "contacts" will tell you where the fighting is. Most fights consist of fighters dog fighting and vulching ( killing enemy as they take off), but in some cases there will be a number of people trying to work together and capture the base. So do what ya like.
Engaging bad guys, those would be the "red" iconed guys, as green is friendlies, is allowde any time. Its good form to call out to a guy who is fighting 1 vs 1 and ask if he wants help. While its a "nice" thing to do many don't both and just grab any kill they can get.
Froming a squad, you get I think 2 guys to join your squad, check in the note book under the officers club I think it is. Joining a squad, you have to be invited by the squad leader. A word of advice, fly with different squads a few times to get an idea on how they operate. They may be pure fighter squad and you might like bombing or visa versa. No need to rush into a squad.
To help figure out all of this stuff a great place to start is
Aces High Trainers Site. Also checking to the training arena is a good idea too. Hooking up with a traing to get you going will make things easier, and a lot more fun !
Welcome to aces high !