How to implement fire detector (smoke+temperature) for an electronics project?
There are several ways to detect smoke. 

The simplest for your project would be a dedicated smoke sensor such as the MQ2. It is basically designed to be interfaced to uCs with 5V inputs. Read the wiki for more examples.



(http://www.seeedstudio.com/depot/grove-gas-sensormq2-p-937.html)

If this is not a good example consider using an IR photodiode with trans-impedance amplifier to detect fire. You can also just buy a home fire/smoke detection, take off the cover and wire it's outputs into your uC (probably through some level shifting circuitry.