Loosen the base cover so it drops down, loosen the fan, then twist all the wiring connectors. Install a light fixture. Don't leave the wires unused as it could cause a fire.
First, turn the fan on then turn off the electricity at the circuit breaker until it stops running. Use a non contact electricity detector if you have one. Loosen the fixture and find the wires to the fan. Loosen those wires and cap each one with a wire nut. Put everything back in place and turn on the circuit breaker.
Before you do that, please PLEASE turn off the electricity to that room and in particular to that ceiling fan! If you are a complete newbie to dealing with electricity, give an electrician a call. No amount of money is worth your life. You can then watch the electrician and learn.
Thank you for the suggestions. I’m going to use Tom’s technique.