Flight sim script help

I would like to modify the player.cs script to do the following:
The player (in this case a jet) has 3 speeds -- slow, medium, and fast.
When you press forward once, the speed jumps to slow. If you press forward again, the speed jumps to medium, etc.
I would like the jet to retain its speed (without the player needing to hold any buttons).
And I would like the arrow keys to control the jet instead of the mouse.
Also, I would like zero gravity.

I'm an inexperienced scripter/programmer, so I'm having trouble trying to accomplish these things.
If you can just point me in the right direction... Or write out the script for me, I'd be very grateful.