If you're planning on using it right away, buy an avocado that yields to soft pressure, but isn't squishy. You can buy a slightly harder one and leave it on the counter to ripen. You can stop the process by refrigerating! We put our soft avocados in the fridge and the unripe ones in our fruit bowl. Sometimes it's hard to tell if it's overripe until you cut it open and see brown spots - a lot of the time I'll still eat them though. They're fine for guac at that point.
Re: how to tell ripe vs. over-ripe avocados?