What should I know before getting a dog?

Backround info – I'm twenty and only really had cats before, but have dogs at for my sibling. I'm also Agoraphobic w/severe Social anxiety/general anxiety, but have felt better when I'm walking a dog [planning on dogsitting my sisters dog mpre often to see how I do] and I'm not willing to live alone without a dog for protection so I've been thinking about getting a German Shepard. I have twoish cats, ones just out of kittenhood and the other is like seven years old, the other is 18 who we cat sit for my aunt.

I don't really know what I need to know to be able to care for/own a dog so if you anyone can help me out with what I should learn/what sites to use/things to know for getting a dog that would be very helpful. I'm planning on doing a bunch of research and want to keep a long tllist of everything I've looked into.

Thanks in advance!