First, determine that the diver is, in fact, in need of assistance. When you find a diver face down on the surface, you should tap him or her on the shoulder to see if you get a response.
If there is none, treat them as unresponsive. Establish positive buoyancy, and assess the diver from there. You may need to call for help.
If the diver does respond, try to determine if he or she needs help. If you are responding from a boat or shore, you will need to establish buoyancy by throwing the distressed diver a float. If responding from in the water, inflate the BCD and/or drop the weights.
Calm the diver by talking quietly, getting the diver to relax and breathe deeply. And if necessary, assist by doing a tired diver tow.