The squirrel monkey is a native to South America where they inhabit the forests of the country. They are an EXTREMELY social animal banding together in troops which are usually extended families.
I feed my squirrel monkeys a diet of almost exclusively Purina Monkey Chow (soak this in water for the babies). I've found this to be the best basic diet available. I also give fruit (especially citrus). Citrus is extremely important since primates do not synthesize vitamin C. Do not feed exclusively fruits because they will not eat the monkey chow.
I strongly suggest a minimum of 6 ft x 6 ft x 6 ft. These guys like to run around a lot and will use every surface in their cage, so use several branches that will not break or give way when they land on them. Squirrel monkeys are extremely social and will become very neurotic if they don't get lots of social contact. If you can't spend a lot of time with them, get 2 or don't get any. They also need something to do. They are quite intelligent and will "go crazy with boredom" if they don't have some "toys" to play with. Be sure to provide a den that is heated in the cold weather. Squirrel monkeys are pretty hardy and can put up with cold weather (below 40 degrees) but they are not from the Arctic Circle, they're from the rainforests.
I have found the best cage material to use is rabbit wire, 1" x 1/2." This is strong material that will support itself and keep most small children's fingers out. Additionally, I strongly recommend raising the cage at least 2 feet above the ground to minimize parasites.
Standard worming for dogs or cats apply. Also, be sure you have a vet that knows about squirrel monkeys, an emergency is not the time to look for one.
Go on the advice of a good vet who knows primates. Vaccinations depend on what is endemic to your area. You may also want to consult with your nearest zoo.
This paper is by far only an introduction to the care of squirrel monkeys.