If the cat is staying at your house (which is not the ideal situation), you should keep it confined to a relatively small area, for instance, a guest room or similar smaller space. You'll want to prepare the room as though you were child-proofing it for a guest with a small child, but one that can jump like a professional basketball player (so simply putting dangerous or fragile items "high" will not be sufficient). For some ideas on how to handle the cat during its stay, look at information about adding a newly adopted cat to the household. Your friend's cat will be in a similar situation, and should be handled similarly.
The ideal situation is for the cat to stay at your friend's house, so that it remains in a familiar space. Cats do not care for changes in their territory, so by leaving the cat in its current territory, you will save a great deal of stress on it. If the friend is expecting you to be around the cat any time you aren't at work, this would require you to stay at your friend's house. You and your friend will have to agree on this type of situation, however, but you should bring it up now so it can be sufficiently considered.
Depending on the cat, constant supervision by a sitter won't always be necessary; they're more independent than dogs (or children), and can be left unattended for longer periods of time. You'll want to consult with your friend on whether there are special circumstances requiring more continual supervision that would require you to share living space with the cat, as well as specifics for its care and feeding. It's often sufficient to have the "sitter" stop by a couple of times a day (sometimes more, depending on the specifics of care; for me, it's three times per day) to feed the cat, clean the litter box, and possibly spend a little time around the cat (if it likes the sitter) to be sure it's doing well and no accidents or illness have come up.
If this is your friend's first time leaving their cat with a sitter, they'll want to leave instructions for feeding (what, when, and where), water changes (bowl, fountain, etc), litter box cleaning (when, where to discard the waste), any medications, and the numbers for both their regular vet and the nearest emergency vet. As you have minimal experience with cats, I would also suggest having your friend walk you through everything so you are more familiar with all the steps before having the cat's care turned over to you.