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You will need a replacement bulb and a replacement gasket. If you don't know what kind of light you have this many need to wait till you have the enclosure open.
Remove the light enclosure from the pool. Usually this is done by removing one screw from the retaining ring underwater, which allows the retaining ring to be removed, which then allows the light enclosure to be removed and lifted to the surface. Hopefully the cable is long enough that you can place the light enclosure on the deck. If the cable isn't long enough things get tricky.
Once on the deck you can open up the light enclosure, replace the bulb, replace the sealing gasket, put the light back in it's niche, coiling the cable behind the enclosure, replace the retaining ring, and screw it into place.
Make sure the power is off for all of this except for a quick bulb test. Only turn the power on for a moment after replacing the bulb to test the bulb. If left on above water the lamp can overheat.
