So with your back foot, you want to kick the tail of your board down and right when it touches the ground you jump in the air off your back foot. This is the ‘pop’. It takes a little bit of getting used to but most beginners don’t really understand that initially, your back foot is doing most of the work. Then bring your knees up to your chest after the jump, the higher up you pull your knees the higher up your skateboard can go!
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.