I can tell you exactly what to do, and you can watch it being done 1000  times, but it all comes down to this: You have to figure out for yourself. Feel it out, this trick takes a lot of trial and error. Relax, you'll get it.

Okay, put your feet on your board. Your back foot should have the ball of  your foot right in the center of the tail, almost all the way back, or the  'sweet spot'. Your front foot goes in the middle of the board, exactly where is up to you. I like to keep mine just in front of the middle, but I push it back a bit for a bigger ollie. Keep your front foot squared on the board, this will prevent it from flipping.

Bend your knees: This is absolutely essential! A pitcher has to wind up for a  pitch, and a skater has to wind up for an ollie.

Okay, here's the tricky part. It's tricky because you are doing a few things at once.

*Jump off your back foot while dragging your front foot up the board. This will cause your tail to hit the ground, and pop the board up into the air. Your  front foot will guide the board up, and even it out at the peak of your ollie.

*Once you are evened out in the air, you just extend your legs to the ground as you drop. Extending your legs helps you absorb the shock. You should land with all 4 wheels at the same time, with both feet over the bolts. This will prevent the board from breaking, when you are ollieing huge gaps.

It may be helpful for you to ollie at first by putting your wheels in a crack  (so you don't roll), or maybe in between two walls you can out your hands on (it  will help boost you up). Do not, however, try learning to ollie in the grass. Sure, it's nice and soft when you fall (note: you will fall, it's going to happen. Falling is just a part of skateboarding, and that's what makes it so rewarding to land your tricks), but when you try to ollie in the grass, you  can't pop your tail off the hard concrete. You need this pop to get your ollies up in the air. Once you get a feel for it, though, learn them rolling, and then learn them rolling fast. It also may help for you to put something out to ollie  over, whether it's a broomstick, a board, or a garbage can.