After you’ve properly applied a burst of force to the tail, the top-side of your front foot’s toe should start dragging up your board almost automatically. Eventually, the higher you are able to bring your front foot during lift will determine the apex of your ollie, so work gradually towards lifting your front foot as high as your body allows. Skateboarders who record the highest ollies usually (somehow manage to!) bring their feet higher than their hips!
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.