How do you change a pull string light fixture to a new light fixture with a wall switch. I have two wires running to the existing string light and am not sure what to do with those wires. I plan to change the light as well.

You might want to hire an electrician for this You will have to #1 pull romex, #2 install a box and switch and #3 get everything installed on the correct leg, as you switch the hot and not the return path.

Basically you need to run a "Switch Loop". I wold suggest using # 14/2 AWG, NM copper cable

The loop takes the power from the main black supply circuit and feeds the switch via the retasked neutral conductor. You take some electrical tape or a sharpie and cover the white conductor to mark this conductor as a retasked conductor (from Neutral to Hot).

I prefer to connect the retasked hot the bottom of the switch. The black conductor in the cable is connected to the top screw of the switch and is used to return the power to the fixture.

The Neutral in the ceiling will connect to the Neutral on the fixture.