Hi Ellie,
I would get a book from the library that explans different breeds and how they react. Of course not every lab will be gentle and not every sheperd will be aggressive. It really depends on how they were raised and treated. If the poor dog was abused it will be more difficult to live with and train. A lot of dogs are given to shelters because the owners die or move, not becasue the dog was bad. Getting a book that explains different breeds would be my first step.
Good luck and have fun.
Another thought, perhaps a senior dog would be a better find then a puppy.
Puppies will pee in the house and will chew on the furniture, not all but most.
A dog is not perfect, it's like another child, patience is the most important thing, all dogs will make mistakes.