There are many ways to prepare an avocado. Here is a good website.
The flavor is often very subtle unless the avocado is a very good one. You should buy unripe ones, ripen them at home, and eat only the ripened ones (website talks about this). Taidong has some good ideas above, but sliced avocados are also really delicious just peeled, sliced, and with a little salt and fresh lime or lemon juice squeezed over. Stab 'em with a fork and enjoy.
Hmm, just talked myself into a trip to Costco.
ETA: to ripen, enclose them in a paper bag (or any bag that doesn’t let in sunlight). Check them daily, maybe twice a day if the heat returns this summer. They’re ripe when the surface just holds a dent from squeezing with your fingertips.