The foundation of any respectable guacamole is a ripe avocado. A ripe Hass avocado, one of the most common varieties available year-round, will give ever so slightly when you press it gently and its skin will turn deep purple to black as it ripens. You can store ripe fruit (yes, avocados are a fruit, not a vegetable) in the refrigerator for a few days.
Unfortunately, the avocados at my market are usually rock hard or verging on rotten. So, I’ve gotten into the habit of buying avocados that are still hard a day or two before I plan to make guacamole. You can speed the ripening process along by putting the avocados in a brown paper bag with a banana or an apple.