Just before a skater performs an ollie, there are three forces acting on the skateboard. One of these forces is the weight of the rider, shown here with two red arrows. Another is the force of gravity on the board itself, shown with a small black arrow. Finally, blue arrows show the force of the ground pushing up on the skateboard. These three forces balance out to zero. With no net force, the skateboard doesn't accelerate, but rolls along at a constant speed.
To get higher ollies, I would suggesting resting your back foot on the very end of the tail and the front foot in the middle of the deck to allow for more slide. Popping as hard as you can and catching your board in the groove of the nose with you ankle practically resting in the groove will level the board out in the air. after this initial movement slide you feet back to the bolts on the tail and nose to ensure a clean landing.