Hi there
You will need to use an ADC present inside AtMega 32. Circuit for LDR sensor is quite simple; make a two-resistor voltage divider circuit with one of its resistor being LDR and take their point of joining to be the alanog input for ADC pin.
Here is a similar project on EG, check this out:
"engineersgarage.com/microcontroller/8051projects/interface-LDR-ADC0808-interrupt-clock-AT89C51-circuit"
Good luck 