First of all you have to think what you will use to measure the actual temperature.
For instance, you may do it with an LM35 chip. This chip produces an output voltage proportional to the temperature. Since this is an analog variable, you will have to make it digital with an ADC.
So what you really need is to make the interface for that ADC (what are you using, an FPGA, CPLD)?
Then comes the display. Will you display temperature with an LCD, 7-segment display?
The problem is to make both interfaces and glue them together.