The "2" key on the keypad is probably going to be used the most. Hold it down until you get to an edge or something, and let go to jump. Holding it down between tricks picks up speed.
The "8" key grinds. Get to a rail or something and hold "8" to grind on it. Use it in combination with the "2" key to multiply your grind score.
The only other keys you'll need to use are "4" and "6." When you're in the air, hold the arrows in awkward positions and hold either "4" or "6." You'll get some nice tricks off.
Since the demo only features the hangar level, try grinding on the helicopter blades on the next hanger over (break the glass) to open up a secret compartment to an area outside where there's a huge ramp. Get enough air there and you'll get mad bonus points for your tricks. You'll notice that the bonus is called "It's Cold Up Here."
Have fun.