hi
following link will help u interface adc 0804 with 89c51 https://www.freewebs.com/maheshwankhede/adcdac.html.LM35 has Linear + 10.0 mV/°C scale factor, that means for every degree rise in temperature its voltage is incresed by 10mv.u have to provide voltage (Pin 1)and ground(pin 3) to LM35(for TO92 and TO220) ,connect output (pin 2) either directly to 0804 input or use lm358 to amplify the input signal ,u can use following formula to calculate the LM35 output and then process it to get degree centigrade values.
Analog voltage=ADC value*Step size.
step size=vref/256,
256 is used as u will be using 8bit ADC.