Do you know if the cat is a wild cat, or someone's pet?

If it's someone's pet, call them and make it their problem. ;-)

If it's a wild cat,

1) Put some food on the deck
2) Make nice with the cat
3a) If this works, see if you can put the cat in a cage.
3b) If this does not work, borrow or purchase a live trap (about raccoon size should work) and bait it with tuna.
4) Take it somewhere to be fixed. If you do not, you are likely to end up with many more cats than you really want in the neighborhood.

If you take the cat to the "Humane" Society, it will almost certainly be euthanized. There are other options for wild cats; look through your Yellow Pages for "Animal Rescue" entries. Even if one of these places can't help you directly, they will probably know where to send you.

Note: We have had at times 28 cats under our care, 27 of which were wild, semi-wild, or the progeny of wild cats. Originally, there was only one wild cat living under our deck... so I speak from experience. ;-)