Well, I typed all this up while Johnny L.A. snuck in there, so I’ll still post it. One can never have too much avocado information. 
Find one that has no obvious blemishes. If you want to eat it today, find one that’s got about as much give to it as the tip of your nose when you push it with a finger - that is, it should be soft, but not TOO soft. If your thumb splits the skin when you gently press, it’s way overripe, put it back and ignore the produce guy scowling at you for smooshing his nasty rotting avocado. If they’re all hard, that’s okay too, but you won’t want to eat it today. Just bring it home and leave it out on the counter (in a bag if you want to speed things up) and give it a gentle squeeze each day to test for ripeness.
When it’s ripe, it’s time to cut it open. The first thing you’ll notice when you try to do this is that there’s a huge pit in the middle, so you can’t slice all the way through. Just run your knife all the way around the avocado lengthwise. Then twist the two halves, and one will pop off the pit. To get the pit out of the other half, hold it firmly in your non-dominant hand (my left) and gently but firmly thwack the pit with the sharp edged blade of a sturdy, heavy knife. I like my French blade for this. It will stick in the pit, and now you have a handle with which to twist out the pit.
Now you have two halves with thick leathery skin on them. The skin is inedible. The easiest way to get the flesh out is just to scoop it with a spoon, scraping the skin clean on the inside. If you want pretty slices like on a restaurant salad, you can make parallel cuts through the flesh all the way to the skin, and sort of turn the skin inside out - the slices will pop our more or less intact (more when you’ve had some practice at it.) Similarly, you can cube the flesh by making parallel and them perpendicular cuts, again all the way down to the skin, and popping those out the same way. Oh, hey, look! A visual guide! The way they do slices looks like too much work for me, but it will certainly work
Avocado doesn’t taste like much on it’s own. A sprinkle of salt and a drizzle of lime juice will wake up the flavor.
Avocado will turn brown when exposed to air. Old Wives tell us that keeping the pits in a bowl of guacamole will prevent that, but I’m sorry to say that this is one time when the Old Wives are full of it. If you want to prepare guacamole or sliced avocado ahead of time, just wrap it tightly with plastic wrap touching the cut or mashed surface, and that will help keep it that lovely ugly green color.