Reddad,
Don't know your location as it's not in your profile, there are lots of events you can participate in. Here's a list.
Wednesday Snapshot - (Time favours US based players) - Walkons allowed.
Friday Tour Of Duty - (Time favours US based players) - No Walkons - Players must preregister with a squad. There is usually lots of opportunity to do this. (squads are obliged to offer at least two guest spots)
Saturday Combat Air Patrol - (Time suits US and europe) Walkons allowed - Just arrive choose a side and the CO of that side will assign you to a squadron. Might be an idea to go with a squad who fly in the Tour Of Duty, if the squad get to know you you should have no problem flying with them in the TOD.
Sunday TOD (Time favours europe) Same rules as Fri TOD.
In the absence of a TOD (TOD's are three weeks on 1 week off) there will often be a once off walkon event like a snapshot.
Scenarios are planned well in advance and are pre-registered, however walkons *may* be admitted if to fill slots left vacant by no-show registered players. To show up, *many* folks who show for these events do get some kind of slot.
Reddad, I have found special events to be infinitely more fun than the main arena. The whole idea of having 1 or at least a small number of lives does wonders for the adrenaline flow:)