While riding up the ramp, shift some of your weight back toward your rear foot, and right before the front wheels reach the edge or coping of the ramp, you want to smack down on the tail of the skateboard and pull up your front knee. This is just like a regular ollie - pull those knees up tight, roll your front foot, and guide the board up into the air as your clear the ramp.
Depending on how high the ramp is, you may fall a little ways before landing. That's OK - enjoy it! Just keep your knees bent and flatten out the board beneath you. When you land, lean into it towards your front trucks and bend your knees deep. Land all 4 wheels at once, let your knees take the hit, and ride off!