Use the following controls while in the Mako.

Press RB: Fire rockets.
Press RT: Fire main gun.
Press A: Undercarriage Rockets. This is useful for getting "unstuck", setting the Mako upright if upside down, and stabilizing from falling off a mountain.
Press X: Return to the Nomandy.
Press B: Exit Mako for on-foot exploration.
Hold LT: First person view.
Click Right Analog-stick while pressing LT: Zoom. Click it again for additional zoom. Click it again to exit zoom.