The best way is to keep the new cat in a separate room when you first get it. The other cat will know it is there by the smell and sound. After a few days, put the old cat in a room by itself and let the new cat out to explore around your apartment - you can do this for just a few hours. The new cat will leave its scent around and will pick your old cats scent. Cats are very big on marking things.
You can also add blankets like spectrum said with the cats scents on them to rooms.
Put the new cat back in its room and let your old cat out again. Old cat will smell the scents all around that the new cat has left.
The next day ( or pick a day when you can be around and supervise and have time). Open the door to the room that the new cat is in and watch the cats - let them discover each other (don't pick the cats up and plop them right in front of each other). There will be probably be some hissing and maybe some batting. If things aren't going well after a while, you can put the new cat back in its room to give both kitties a break. If things seem to be going OK, you gave leave the new cat out but be sure to stay around to supervise.
You can try again maybe the next day or a later that day. One thing to make sure you do is to not show either cat more attention than the other - don't punish either cat for hissing or whatever. Another good thing to do, once the cats are a little used to each other, is to play with them together. Don't single a cat out and play with one and not the other. The playing together will be good bonding for the kitties and with your family.
Remember that some cats will hit it off immediately and some will take longer - so don't give up hope.
I have three cats and two of them are more bonded than the other - The other one is more just tolerated by the other two and they still get a bit hissy when getting too close to one another -even after 9 years. But as long as they don't tear each other apart, everything should be fine.
When you go and adopt your new cat, ask about a cat that the adopters know get along with other cats well. Some cats are better if they are the only kitty in the house.
Good Luck! Let us know how it goes (post some pictures of your kitties too!)