A simple thermistor being read by an analogue to digital convertor and then the 8052 doing some calculations to convert the readings into a value suitable to be displayed, or you could go for a more sophisticated device such as the dallas ds18x20/ds1822 on a 1 wire bus but thats really not a beginers project.People will help you but dont expect them to post code.The thermistor is probalby easiest for a first time project so you need to google for information on thermistors and on 8052 variants with built in adc which can take the readings from the thermistor.Some people like the lm32 from national semiconductor http://www.national.com/pf/LM/LM35.html because its output voltage is a direct representation of the temperature so its sightly easier to use than a thermistor because a thermistor is a standard voltage divider arrangement isnt linear in its output.