Keep the new cat in a separate room by itself with the necessities it needs. While it's hanging around in that room, it'll give your other cat some time type to sniff the scent of that new cat. As a couple days go by, try to introduce the cats. If there's aggression or tension just move them away and keep doing the same procedure. Sometimes you can wrap a blanket around the new cat and wrap a blanket around the old cat and then switch the blankets so they can get used to each others scent.
If there still seems to be tension, try putting the older cat's food bowl next to the room where you're having the new cat. He or she will eventually get used to the scent while he/she goes to eat.
Don't be concerned if this process takes you about 1-2 weeks. It's normal. However, if it's taking longer than 2 weeks try going to your vet to get a referral to a cat behavior specialist.
And one more thing! If your cats get in a fight don't get in between! Even if your cat loves you with all his/her heart he/she can still give you a nasty wound from being aggressive towards the other cat. If a fight breaks loose simply spray the animals with water.