I'm planning on getting a new dog soon, but my current dog (the only one I have atm) doesn't like ANY other dog unless she has known them for a while, so I was wondering.....
How could I get my old dog to get used to the new dog in the house?

Get something from the dog (chew toys, etc) and gave it to the other dog, to let them familiar with the smell?

Split them up, but make sure they can see each other.
Use a baby gate or something.

Rub blankets on each of them and try to have them smell in the scent of each other. If I'm able to get the dogs together without fighting, I tend to pet them at the same time to show I show no favoritism.

Let them interact provided you can step in if things START getting out of hand (if a full on fight starts it can be dangerous to intervene.) Your current dog will probably want to assert dominance, and tje new dog may or may not accept this. If you introduce two very dominant dogs, you may be setting yourself up for a lot of problems. My suggestion would be to find out about the new dog's history, whether it has grown up around other dogs and whether it's typically 'alpha dog.'
All our dogs have always been very good with other dogs and other animals depending on our reactions. We've never had problems introducing new dogs to the family, as well as other animals, as our current dogs generally base their behavior off ours. Seeing us interact positively with new animals generally leads to our dogs following our lead. Sometimes they will scuffle if the newcomer doesn't 'follow the rules,' but other than that we have no problems. It all depends on your dogs personality, training, and its relationship with you, so you will probably be the only one who can come up with a way to introduce the two dogs without too much hassle.

Call Cesar Millan.

"You haf to bee da pak leedarh."

Calm and assertive, as Cesar would say. Dogs need/want/crave leadership and as long as you're the leader, there won't be any serious problems with your pack. When you give up control, then the problems start.

Put both dogs on leads and take them for a *brisk* walk. This gives them the opportunity to "work" together, without any down-time to bicker.

a good method I use is to have them meet on neutral ground first, with both on leashes. I brought the new dog over, and let them see each other, but not get too close, then me and a friend walked both dogs up and down the street. Starting on opposite sides, and gradually getting closer together. This gives the dogs some time to be around each other and investigate things while you are still in control of the situation. Never bring the new dog into the house without a proper introduction. The old dog is more likely to be defensive and territorial if you do that. A neutral area, even if it's just the road in front of your house is much better for introducing them.

My nan recently got a new dog with a dog she had, and the dog she had did not like other dogs. This dog was a big Husky, while she introduced a King Charles Spaniel to him. They do fight a bit, as the Husky tries to assert it's dominance, but they have kind of got used to each other, and play with each other sometimes. Maybe you should have a go at the direct approach, and if they fight, use some of the techniques that are mentioned here. And don't show preferential treatment to any of them, for example, don't take one on more walks than the other, or give one of them more treats than the other. They should get used to each other. And if you give your dogs scraps, share them evenly.

I also recommend this piece of advice. At one point I had to deal with 4 new dogs at once, and getting them used to each other's scent before they met worked wonders.