The commercial growers, when they become ready, when the hulls open and expose the nut shell, use mechanized shakers, heavy trucks with an arm that grabs the tree and shakes it, letting the nuts fall into tarps spread under it.
But the rest of us pick them up as they drop or "thrash" for them. Thrashing is just throwing a stout stick into the tree repeatedly to knock the nuts loose. I know a number of Texas cities with ordinances criminalizing thrashing of trees not your own.
The squirrels will do considerable damage, especially at times like this when the drought hasn't left them much else. They will take a bite out of a green pecan and drop it. They don't seem to know when they're ready, or they're just bored. Pecan pie makes a good dessert following fricassee of squirrel.
The shell situation depends. There are native pecans that are small with tough shells. Very good meat, but hard to get at. Papershells and such are easier and can be done by hand. Or, you can find out where in the area someone has a machine for cracking pecans. The machine takes them all in and shatters the shells, making it easy to get the meats out. They normally work on a split. They get some percentage of the nuts. If they're not impossibly tough little natives, an impact cracker works well. Like this rubber band driven one:
You can get some folks to help, but you have to stuff them pretty good first, or they'll eat as they shell, and the pecan will end up in them instead of the bowl. Freeze the meats for future use.
If they turn out to be good pecans, learn to take care of the tree, how to prune and fertilize. A pecan tree take a LOT of water to stay healthy. And be real alert for bag worms that will build a web nest and then hatch out to eat the leaves.
A top producing variety of mature tree at peak productivity can produce up to 500 pounds under ideal conditions. Mostly, it's much less and quite variable from year to year. The good news is that a good tree can produce for 100 years, even 200 years if well pruned and cared for. Pecans were terribly expensive last year. The Chinese had discovered them, and they were the latest big thing there. A bit cheaper this year. But always enough to be worth gathering.
If you have a dog, and the dog discovers the crop, they'll eat themselves full of pecans.