The Ollie is the basis to basically every other street trick in skateboarding, you should practice your Ollies and get them good before you do any other trick. If you dont then any other tricks you learn, like a kickflip for example, will be harder to learn and you wont be able to do them properly.
First of all put your back foot ( either left or right depending on which is more confortable for you) on your tail. When you first practice your Ollies its better to put your front foot just behind the front truck bolts instead of in the middle of your board. When Ollie-ing a few things need to happen at the same time. Bend down and quickly snap down with your back foot (this is called ‘popping’) and jump up. Your front foot should slide up the board, if not then try again but force your foot to turn on its side and slide it up. It takes a lot of practice for Ollies and they will become higher through the months and years of skating. If you can get a good technique for an Ollie then you will learn them faster and get them better. You MUST learn to get these right before trying to learn any other trick as it will make your other tricks small and ugly if you have a really small or bad ollie.