Sourcing and storing avocado
I usually buy green, hard avocados to use them in a couple of weeks. When I'm looking for an almost ready avocado for my guacamole at the supermarket, I look for a not-so-hard one and pop off the stem: this is a tip I found over the internet and it really works. Pulp under the stem should be greenish and not brown or dark, otherwise it's probably rotten. You can usually find your ready-to-enjoy avocado with a couple of pop offs.

Take a look to this link for a more in-depth explanation about popping off the stem:
http://www.nwedible.com/2012/05/never-buy-a-rotten-avocado-again.html

Regarding storage, I put them in a bowl on counter top to let them ripe slowly; in a bag with apples or bananas to speed up things. Once ready, I store them in the fridge. Once opened, I put them in Tupperware: they stay perfectly green.

Keep in mind that if you place fruits near apples or bananas, which naturally produce ethylene, your fruit will generally ripen faster.