I knew a guy who never got any better than absolutely awful at Halo 2. He played on a fairly regular basis and it was seriously like watching a toddler play. I don't think he ever once killed anyone ever. He'd constantly jump or walk of the map or blow himself up with his own grenades. What made this especially hilarious was that he would yell and scream at the top of his lungs. And this happened ALL the time over a fairly long period of time.
Someone who's never played Halo before being unable to figure out how to drive a Warthog? That's nothing.
But yes, make sure you're in the driver's seat and you use the left stick to go toward and away from whatever direction you happen to be looking in. It's honestly kinda weird since just about every game made in the past 10 years uses the RT button to go forward whenever there's a vehicle to be driven.