What you do with the kitten all depends on two things.
1. What are your long term plans for the kitten?
2. How old is the kitten?

If you don't plan on actually keeping the kitten, then the best thing to do is take it to a Vet or Shelter so that they can care for it and find it a foster home if it is still too young to be rehomed.

If you plan on keeping it, then the best thing to do is take it straight to a Vet and find out it's age. There's been lots of advice in here on what to feed it, but that is all dependent on how old the kitten is. If it's really young it'll need syringe feeding with a milk supplement (like divetelact that someone mentioned), if it's a bit older itll be able to drink the divetelact by itself from a bowl, and if it's a bit older than that then the best "first food" for a kitten is a good quality (bought from a Vet) kitten dry kibble that has been softened with water (steer clear of things like weetbix and milk, they will cause tummy upsets and don't have the nutrients a kitten requires). Based on the kittens age the Vet will be able to tell you if you need to help the kitten urinate and defecate. This sounds really gross, but it's not too bad. You just just a cotton ball and wet it with warm water, then wipe it across the kitten genitals and that stimulates it in the same way as a mother cat licking it.

The Vet will also advise you about worming and flea treatment. Spot on treatments for fleas are safe from about 8-9 weeks of age, but there is a Frontline spray that can be used from 2 days of age to get rid of fleas. Don't use a supermarket brand! They use old technology and there are some that have a chemical in them that can cause convulsions and paralysis (and death).

If the kitten is from a feral cat there is a possibility that it may already have FIV, but as it may not you'll have to keep it separated from your other cat. There is an FIV vaccine that is extremely effective. You can have the kitten tested for FIV at a later date (after it's been quarantined and you know it hasn't contracted it during that period) and if it's comes back negative then you can get the series of FIV vaccines. A study where the FIV virus was injected into cats in a high dose showed the vaccine to be at least 70ish% effective, but a much more "life based" study where vaccinated cats were placed in enclosures with FIV positive cats and allowed to interact normally showed the vaccine to be much closer to 100% effective.

If the kitten is really young it shouldn't be too hard to keep the cats apart as the little one just needs to stay in a heated box or something similar. If it's older and moving around, a great place to keep it (although it sounds strange) is in the bottom of a shower. You can make it up into a lovely cat enclosure temporarily. Of course, that only works if you have a spare shower that you can convert for a period of time, lol. Otherwise, your Vet may hire enclosures that you could put somewhere away from your other cat. The clinic I used to work at had a number of large enclosures to hire as they were often used for keeping cats or dogs confined after surgery which required that animal keep movement to a minimum.

I guess whatever you decide for the kitten, my advice is that a trip to the Vet asap is required either way. Good luck!!!