Try not to feed cats who come into your garden. This will only encourage them to keep coming back and they may have owners feeding them too or have special dietary requirements due to health issues.
Put a paper collar on the cat asking an owner to contact you if the cat is theirs. If you get no response after a couple of days you can assume the cat has not been back home or may be stray
Cats roam over wide areas so ask your neighbours if they know who the cat belongs to. Create a found poster and put it up around the area where you found the cat, share a photo on social media, local community apps and on any lost cat websites.
NEVER GIVE OUT THE LOCATION, OR PUT PICTURES UP.
Report to your local rescue centres and vets in case the owner has already reported their cat missing and they can be reunited.
http://www.animalsearchuk.co.uk/
http://www.nationalpetregister.org/
http://www.petslocated.com/
http://mymoggy.com/
If the cat is approachable take them to your local vet/rescue centre or scan angels where they will be able to scan for a microchip with the owner's details.
If you find the cat's owner please remember to inform those who are still looking and take down any found posters.