We have all been shown how to place toothpicks in an avocado and suspend it in a glass of water, if not you can find many websites that still tell you that is the way to grow avocados. This abuse of the avocado seed started in the elementary school science class and has been passed on from generation to generation and now with the internet this abuse may never end. The science class was teaching germination, not how to grow an avocado.
How to Grow an Avocado Seed
Plant it in soil point up. If you plant it in the sun you will need to have it under the soil enough so it doesn't get sunburned yet shallow enough to get the heat to aid in germination. Keep it moist not soaked in well draining soil.
Now that it is Growing what to do You now have a seedling. Do not expect to enjoy the fruit from it. Less than 1/10th of 1% have a fruit worth marketing and it may take 20 years or longer to get 2 or 3 fruit. Plan to graft this tree to a variety of avocado you like or you can just enjoy the foliage.