How do you wire a light when the power is taken to the light

Are you talking about installing a switch? If so, you will need to install a "switch loop. If not, Black to black, white to white, ground to light base. Let us know what you are trying to do and then we can help you better.

Your description is very vague.

You would use a switch loop as described by Ma0641 if the supply is in the ceiling.

Make sure you turn off the breaker that is supplying the circuit you are going to work on before you do any work!

From you question, I'm going to guess that you are pulling the power from a receptacle or other source.

If that is true, then you would use a 14/2NM cable to go from the receptacle to the switch. You need to strip off enough of the outer shell of the cable (in the area of the switch) so that you can connect the grounding (bare) wire to the switch (under green screw) The black wire you would cut and strip back the insulation. Connect the black from the source to the bottom of the switch and then connect the other black to the top of the switch. Do not cut the white conductor.

At the ceiling, you would connect the black conductor to the black wire from the fixture. The white conductor from the switch goes to the white wire from fixture. The grounding wires all get connected together.