How do I start an avocado tree from the pit?
No fruit from pit
If you want to eat avocados from the tree you grow from a pit, forget it. Most fruit trees are hybrids and/or grafted onto root stock chosen for certian possitive characteristics. The plant may grow from a pit, but it will not produce fruit. A gardner told me recently that even nursery grown avocado trees have problems because the male and female flowers open at different times, one at night, the other during the day. That's why my flowers just fell off. I'd have to go out at night to collect the male pollen and put it on the female flowers during the day. Next year I will try that.
If you decide to buy a tree to plant, plant it where the trunk will be shaded. It can "burn" and kill the tree if exposed to direct sun light. It also requires lots of water. Nights may be too cool and cause flower drop, too. There are good articles on the care of avocado trees on the web, do some further research for night temps, feeding and watering needs. I got my info from them and some local gardeners.
If you want the plant for decoration, just get a container a bit larger than the pit. Put a few tooth picks a little into the flesh and submerge the pit 1/2 in water. Make sure that the pointy end of the pit is facing up. Keep in warm, sunny place and water at constant level and you should have a shoot and roots in a few weeks. I see that several others have given you the same advice. I used this method as a kid, and kids successfully use it in schools all the time. Ask at the supermarket if they are throwing away any avocados and get one. The pit should produce roots more quickly if the flesh is brown and mushy.