Things to have before adopting: bed, bowls, food, treats, leash, poop bags, a vet picked out, toys. Also decide if it will be crated or not, if so a crate.
You’ll probably want to look at your house with dog proofing in mind especially if you get a bigger breed or puppy. There are lots of everyday items that can make a dog sick or kill them (chocolate and grapes as an example) it’s good to find a list and make sure you don’t have anything poisonous out.
There are sites with quizzes to help narrow down a breed if you have no idea what you want they’d give you a starting place.
It’ll take some time for everyone to adjust, 6 months ago we adopted a 2 y/o GSD that had bounced around houses and probably been abused a little. The first few weeks he was super touchy about his space and would snap, we think someone choked him because he wouldn’t let you mess with his collar. He has slowly settled in and a lot of his fear is going away.
All of that to say if you really want a dog don’t give up if the first few weeks/months are bumpy.