A Nollie is a Switch Fakie Ollie.
First the feet position: Your normal front foot on the tail and your normal back foot in the middle of the board. This trick is harder to learn while moving so learn it standing still then while moving. With your foot that is on the tail snap it down as hard as you can, and your foot that is in the middle drag it up. This is a lot like learning to Ollie so if you can't Nollie to well then think about a Ollie, or read my Ollie page. Just be in Nollie position. When you learned to Switch Ollie Nollies should not be all that hard anymore. Just remember to snap your foot that is on the tail down as quick as possible and remember to drag your foot! To Nollie higher you need to remember to pop as quick as possible and drag your foot. Bend your knees when you are taking off, and expand then when popping. I can't say much more about this just remember to practice.