Pets: If you have a pet dog, (not recommended, but a common situation), we will help with procedures to have the dogs get along. Your pet dog will probably have special privileges (being allowed to go on the furniture, perhaps), and your Service Dog will have different rules, but dogs are okay with that set-up and will soon learn that different rules apply to them. Pet dogs get pet privileges, Service Dogs go by stricter rules. But that's okay--dogs like to work, and they like having clear boundaries and rules.
Dog parks: You really don't want to bring your Service Dog to a dog park. Dog parks are for pets, not Service Dogs, because your Service Dog is trained to work (and not to play with other dogs). Note that you will still want to play with your Service Dog when the dog is not workng, just not at a dog park. Basically, you want to want to continue the training of having your Service Dog to focus on you, so your Service Dog will not be looking to other dogs for attention and fun.
Friends' dogs: Likewise, you wouldn't want your Service Dog to be playing with friends' dogs. If you have to go over to a friend's house and they have a dog, it's best to leave your Service Dog at home because otherwise, the dogs would want to play, and you don't want to encourage that behavior. If there is a special situation (like you are going over to one person's house every single day) then maybe an exception might want to be made, but in general, you don't want your Service Dog to be playing with other dogs, but rather, you want your Service Dog focused on you.