You are looking to make a voltage divider with your photo-resistor as part of the divider. As the light lever changes the voltage drop across the two resisters will change.
So if you have the divider:
+5 ----- Fixed Resistor ----- Test Point ------ Photo Resistor ------ Gnd
Play around with the fixed resistor, try 10K, 47K, 100K.
Watch the values you read from your Test Point. You will be safe to play around.
See if the value goes up or down as you vary the light. If they are going in the wrong direction, reverse the fixed and photo resistors. That will cause the values to go in the other direction. Try it and see.
Now as you write your algorithm, do you want to peek you value or dip it? They will both work but what will make your code the simplest and easiest to understand? So in your code do you want to think "the higher the number the brighter the light" or "the lower the number the lower the tracking error"?
Experiment, play around, have fun. If you don't understand what you are seeing, ask.