how can fix my zanussi fridge, it frosts up in the freezer?

how can i fix my zanussi 'frost free' fridge, the food compartment does not get cold and the freezer compartment below frosts up. the fan runs continually. if the thermostat is broken how do i replace it and likewise any of the sensors, and where are they and what do they look like ? the model number as far as i can tell is N121, although this may well be a guess on my part !!