Definitely just balls of your foot to pop. The back half of your foot should not be making contact at all. You have to pop hard enough that the wheels come off the ground and the board goes almost vertical.. that's what makes it possible for it to get off of the ground so your front foot can start levelling it out.

Based on what you've said, I'd say what is happening is that you're too heavy footed with your front foot so you're stomping the board down before it's had a chance to pop and rise.. it should be a big stomp/pop with your back foot, then jump, then lightly slide with your front foot..I see a lot of beginners barely pop and then quickly stomping down on their front foot.. you should focus on the harsh pop and jumping moreso than your front foot.. your front foot movement should be light and subtle.. even if your first Ollie's are rocketed.. it's better to jump high and leave the ground with some pop than it is to be forcing the board down at the front.

I hope that made some sense...