Ok, first off, the ollie is the most basic trick there is. It is the foundation of your skating. If you can't ollie, forget about skating. Practically every trick from here on will involve an ollie.
Now that that's over, here's how you do it: You want your back foor right on the edge of the tail, and your front foot around the middle of the board. Now here's the tricky part: You want to bend down, then kick down sharply with your back foot, and, at the same time, kind of "jump". Now slide your front foot forward until it reaches the nose. Then you want to level out the board by putting a little weight on your front foot. It's hard, and it takes practice, but don't worry: you'll have it soon enough.