Photoresistor Transistor Circuit
In this circuit, the centre voltage divider is creating a voltage between 5 to almost 0, where the amount of light shown on the photoresistor determines this. This voltage changes the amount of current going through the transistors, which switches them on or off. The LED will turn on when light is shown but it will also set the arduino pin to low, which is inverted to whether light is shown or not. So you may need to change your program a bit. You can omit that 10k resistor if you set your pinmode to INPUT_PULLUP. This circuits only uses one IO pin which is that unconnected node on the right.

Although I rather prefer MOSFETs, you don't need to use them.