The ollie is a fundamental trick in street skateboarding, and is used to leap onto, over, or off obstacles, or over gaps of unfriendly terrain such as grass or stairs. As so many other tricks depend on it - for example the kickflip and heelflip - the ollie is often the first trick to be learned by a new skateboarder. The ollie typically takes considerable practice to learn.
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.