For me, you can't force dogs to be friends. be careful not to force them to be friends because that can break into a fight. It could take weeks or even months (or longer) for them to get along with one another or it might never happen at all!
Make your dog feel that you love him. Pay more attention to your dog so he doesn't feel threatened. After which you can play with them together so they can interact and feel comfortable with you and each other.
What do you mean with different one? Like a cat for example? If it is a cat, than you are better off, when your dog is very young, or not stronger than the new cat.
Getting a second dog? I can strongly recommend to get the opposite gender. Have the dogs meet outside your home, walk them together, and then bring them in your home together.
What else? You won't buy a pet rat, if you own a terrier (common sense?).
Cats usually adjust to dogs reasonably quickly because they can get out of the way and then gradually as they feel more confident they relax when the dog is around.
If you are sensible and calm with the animals they will get used to each other in their own time, but be careful not to favour one more than the other.
When I was at a shelter attending to something a few years ago, I remember a man brought in his older dog to give up. He said the dog's behavior at changed and cited this and that. They kept asking him if anything at changed. He kept saying no. Finally he says they just got a new dog. Well duh! They'd been giving all of their attention to this newcomer and the faithful 'old' dog hadn't gotten much attention and was acting up. It really irked me just listening to it. What a jerk that guy was.
Orrymain, how could they be so cruel? I feel for both dogs because the new one will probably be dumped as soon as something better comes along.