A pet dog or cat is a 15-year emotional commitment. A little advance planning will help make it a rewarding experience. When picking out a puppy or kitten, don't choose either the most pushy or the shiest one in the litter. Pick out a friendly, happy pup or kitten that will come to you. Then, while it is young, accustom your new pet to people young and old, other animals, and a variety of different situations.
Early socialization is very important.
If you are considering adopting an adult animal that is known to be aggressive, be realistic about what you can expect. Even if the problem was aggravated by the previous environment, rehabilitating an aggressive animal is a big project. To believe the animal needs only tender loving care is a mistake. Animals do change, but it takes dedicated work and lots of time. An aggressive pet is a liability, especially if there are young children around.