Learn how to trap!
Other ways to help:
1) Put out food & water every day
2) Provide shelter, especially in the winter
3) Get them spayed/neutered and rabies vaccinated
4) Check to see if the cat is microchipped, or if there are any lost cat posts that match his/her description.
5) If friendly, work with a rescue to find placement, or open up your home for him/her!
6) If feral, be sure to have him/her ear tipped when being fixed, and return back to their territory outside (and continue feeding and providing shelter).