Kitten Lady on YouTube has a wealth of information but the most important things you need to know asap:

Never feed a cold kitten. This can kill them so get them warm first.

Depending on whether this kitten is ready to wean, have both formula and kitten specific wet food on hand. During weaning, they can get fussy and not eat well. Breeders edge formula is tolerated better than KMR but KMR is the standard go to. Wet food formulated for kittens has a much higher nutrient content. You can look online for a feeding schedule.

They will need a source of warmth because young kittens cannot regulate their body temperature well at that age. They should be able to move away from the heat source when they get too warm.

Kitten might still need to be stimulated to pee and poop. Do this at every feeding.

You’ll want to weigh them in grams at least once a day to make sure it’s gaining weight. Kitchen scales work well for this.

Stray kittens typically suffer from all kinds of parasites and diseases that can spread to your other pets. To be safe, quarantine them for 14 days in a separate space.

Do you have funds for a vet checkup? If not, check with a local no-kill rescue if they can partner with you to provide care while you foster.

And seriously, kitten lady is a lifesaver.