ok first of all u need to get a parachute u will know if you have one in your inventory by switchiong through your weapons list anf it will be to the right of your grenades/bombs section then press R (right trigger, the one you use to shoot) and you should se Luis strap it onto your back (dont jump unless its on your back) then run up to a ledge and once you have jumped press A to use it (you may have to tap it a few times)
Its actually possible to equip the chute while falling. I jumped out of a chopper once without equipping it, and while falling (he still falls awkwardly and yells) i cycled to my chute and was able to equip and use it. Actually makes jumping out of the chopper a little more interesting.
"Your coach is a cheater, your rings are a lie, you were a backup at Michigan." Tosh.0 on Tom Brady