Besides following the advice already posted I would also recommend a quick trip to the vet for a health & age assessment. This allows you to choose a de-flea & worming product & programme. These are the current healthcare requirements for a kitten that is probably 6 to 10 weeks old from your description. Feral kittens tend to be slightly smaller than their lucky domesticated cousins due to the variable nature of their food supply. The other things you need to plan are;
1. Taking out health insurance to cover accidents, allergies & illnesses. Unless you earn enough for this not to be a concern for you.
2. When to have your new charge vaccinated against cat flu, feline lukemia virus, etc. because s/he is at risk after the temporary immunity gained from Mum's milk runs out after about 14 weeks. Shop around your local vets since costs vary & it's a very basic service. This being up to date is essential if you need to put your pet into a cattery for any reason - holidays, re-decorating, moving house, etc.
3. When to have your charge neutered. This can be done as early as 3 months but most vets prefer to do it from around 6 months onward, ask your vets advice. A need for this is to stop adding unwanted kittens to the population & avoiding the health risks associated with breeding. When is as soon as possible but often left until indicated by a female cat coming into season for the first time or a male becoming more terratorial & fighting with the other local cats or starting to scent mark in more destructive ways i.e. clawing & urine spraying.
4. Don't forget to have your precious little identi-chipped so that s/he can always be returned to you. Mostly done at the time of vaccination.
If you are worried about the costs contact the Cat's Protection who will provide low cost or free vaccination & neutering, or the Blue Cross who provide essential vetinary care in your area.
Good luck - x.