When do you want to eat it? If you are looking for today or the following day, hold the avocado in your hand and gently squeeze. Ripe, ready-to-eat fruit will be firm yet will yield to gentle pressure. One thing to avoid are soft pockets. This is typically a bruise which happened during transportation.
If you want an avocado ready in a few days pick hard green fruit.
The best place to store your avocados is on counter in a fruit bowl. It will ripen in 2-5 days. If you want to speed up the ripening process place an avocado and an apple together in a brown paper bag. The apple produces a natural ripening gas which will speed the process.