Since it seems that you're a total newbie, I'll explain it step-by-step.
1. In your chopper's properties (accessed by double-clicking on the chopper in the editor), set the Special field to Flying.
2. Use flyInHeight to make the AI fly at a certain height. Put the following code in the chopper's Initialization field:
this flyInHeight 50
Note that the Initialization field is also accessed in the chopper's properties.
3. If you want your chopper to start with velocity in one of the 4 primary compass directions (N, E, S, or W), it's a simple matter of setting one of the x and z vectors to a positive or negative value as appropriate. I haven't experimented enough with this to know which one is which, but it's easy enough to find out. Try setting one of them to 100 (or some other arbitrary value) and see which way the chopper moves when you start the mission. If it's the opposite way, set it to -100 instead. If it's the wrote axis, then set the other one. Simple, huh? All it takes is a little experimentation. If you want to do a direction in between one of those four compass directions, then you have to do a little math to figure out how much you need to set both vectors. Naturally, you'll also want to make sure our chopper is pointing in that direction at the start by setting it's direction in its properties.
Not too hard, huh?
Just read the tutorials on this site, as well as the ComRef, and you'll find the answers to many of your questions.