Please remember to keep your foster kitty separate from resident animals for 10-14 days before beginning introductions. The overwhelming majority of cats can learn to coexist peacefully if we work at it and give them some time. You'll want to keep the cats separated for the time being. Each day it’s ideal to increase their exposure to one another in some small way. The first step is to let them smell one another, but not see each other. This often means keeping them in separate rooms. I recommend you take a towel or blanket and rub it all over the new cat. Then, place that towel in with the existing cat. Do the same for the existing cat, so both cats have bedding that smells like the opposite cat. When both cats have stopped reacting poorly to that smell, move forward with the next step. This may take a couple of days or a couple of weeks.
If it’s possible to let them smell one another under a door, this would be a great next step. It’s best if they can smell each other, but not see each other. Now is also a time to start using Feliway (a cat pheromone spray that helps calm anxious kitties and can be found at any pet store or online). Spray the Feliway in the specific area you are now asking them to share, the bottom part of the door, in this case, if this scenario applies to your home.
You can now also start making that shared area a really fun place, by giving each cat treats in that spot, feeding them near the doorway, using catnip there, yummy canned food, etc. You're starting to teach each cat that good things happen when that other cat is around. Once the hissing, growling, etc have stopped, we can again increase exposure to one another - this time allowing sight but no physical contact. Again, spray the area with Feliway. Make the area where they can see one another the Fun Zone and feed there, continue offering treats there, etc. When they have stopped hissing and growling at the sight of one another, it's only then that they can be allowed physical contact. Start small and offer short intervals, then let each cat go back to their safety zone. Reinforce the treats and Fun Zone plan where good things happen when they are near one another.
One quick note on this is to remember not to force the cats to need to eat near each other. It is not a natural behavior for cats to eat near each other, they would much rather eat at a bit of a distance from other kitties. This doesn’t mean you can’t feed them within eyesight of each other, just make sure bowls are not side by side.
Very gradually, through slow exposure and through teaching each cat that the other's presence means treats and fun, both cats should relax and learn that they can share a habitat without feeling threatened. The use of Feliway should help significantly and you may want to use it throughout the whole process.