MX vs. ATV Alive

Very easy here, head to one of the free ride stages, find a big ramp, and jump. You should get either a medal for Hangtime, or Jump Distance (or both!) You can also try freestyle jam by pressing Up on the D-Pad. Freestyle Jam is a mini-game where you have 1 minute to rack up as many points as you can by doing tricks. There's a certain goal you have to get to.

Another option is to go off a ramp and get a big combo doing tricks. You can check the goals while in Free Ride by pressing start, then X.
Edit: If its difficult doing tricks, the easiest trick to do is the whip. When going up on a ramp, move both of the sticks left or right.