This is a pretty vague question. Are you keeping the cat while you try to find the owner? Do you have a vet you can take it to, to find out if it's microchipped?
Call your local animal shelter and humane societies – most have a computerized lost and found service (check our Area Shelters web page).
Take the cat to the local animal shelter or veterinarian to have it scanned for a microchip. Then you can call the microchip company and obtain the name, address and phone number of the owner.
Make “FOUND CAT” flyers with a picture and/or description of the cat, approximate age and where it was found and a phone number.
Watch for “LOST CAT” signs in your neighborhood.
Check the newspaper daily for “lost cat” ads that may describe the cat.
Place a “Found” ad in your newspaper – most papers will run these ads for free.
The only thing I can add is to call your local radio station to see if they have a free lost and found for pets. If not, most have a free 'swap shop' or something similar, where people want to buy or sell things, and I have been on those with great luck. If you can't find the owner, then you could put the poor little lost soul on to 'give away' and give them info on how the poor little thing needs a home right away.