Make sure it's clean, get some Neosporin on the cut, put some sterile gauze over it, tape it.

Depending on the depth of the cut, you might need stitches.

I raced with a guy who cut his feet (both of them) on oyster shells - we were pushing a boat out on low tide thru mud and wanted our shoes dry, so we went barefoot. I was lucky, he was not, but he ended up racing for the next seven hours on bad feet.

When he figured something was wrong with this feet, we had already reached the summit and simply had to turn around. We cleaned his feet on the course, used some Neosporin, and duct taped his feet. Worked great for me, I didn't feel a thing :)