The seeds will grow into a nice tree, but it is uncertain what type of fruit the new tree will produce, even if that seed came from a great tasting fruit. The best way to propagate an avocado tree is through grafting.
Most commercial avocado trees are grafted. The grafting material, called a scion, is basically a green twig taken from a tree of known, good quality fruit called the mother tree. This scion is then grafted onto a young seedling, perhaps a year old. All the fruit that is produced from this new tree will be identical to the fruit borne on the original mother tree. When a grafted tree is purchased, the exact type of fruit is known. On the other hand, when planting an avocado seed, the type of fruit that will result is unknown due to gene mixing. In fact, it is through the gene mixing that new varieties are discovered. It is possible to produce a high quality fruit, but unlikely. What usually occurs is the seedling tree will produce small fruit with a large seed.
For instructions on grafting, go to the University of California website, http://anrcatalog.ucdavis.edu, and search avocado. There is an excellent publication called Budding and Grafting Citrus and Avocados in the Home Garden.