You will need to stand on the board with one foot on the tail (back end of skateboard) and your right foot in the middle or behind the front screws of the board. Really you have to find the "sweet spot" to get the most height when doing an Ollie.
Now for this step you have to preform two actions at once to make the board go into the air. The first step is pop the tail (back of the board) down. For the second step you will have to slide your other foot upward to bring the board into the air.
This step of the Ollie might sound the easiest but really when in the motion of doing an Ollie you don't think to it which will not allow you to get the most height out of your Ollie. After popping the tail down and sliding your foot upward you will need to bend / bring your knees up. This will allow the board to go even higher.
Finally you have to remember not to be scared and lean into the Ollie. If you lean forward it will make things easier when trying to learn how to do a good Ollie while riding. When you lean forward it'll actually help you control the board. If you don't lean forward you wont get the best Ollie you can.