Good on you for considering, the most important thing to do is make sure you have a space to isolate, even if they are going to be the only cat - if you get a stray cat, you want to make sure it is flead and wormed before being loose in your house, one room is much easier to treat than the whole house, and you might end up with a nervous cat who needs a small space to get used to things first, and it should be as cat proofed as possible, although they are very good at finding spaces you didnt think were possible - I have fostered for 4 years now, and the last lot of kittens got in spaces the others hadn't. You also have to be able to give them up, which can be hard, there are some cats that get under your skin that bit more than others.
Each rescue is different in what they will provide for, and how they treat fosterers. I have fostered for 2, and i prefer to foster for a smaller, lesser known charity, as they didn't have the set up, so I know I made more difference to cats with them. You have to be able to give them enough time on a daily basis, as well as the cleaning that they create.
