Hard to answer with almost nothing to go on. Look up LM34 & LM35 devices. They have linear outputs in metric and Farenheit units. All you have to do is amplify the signal to suit the A to D input (assuming the unknown PIC you're using has one).
There are quite a few thermometer circuits out there, some using AD display converters, others using PICs. In fact, I am making one now for my fish tank. Have a look on google there are some nice circuits out there.
The Dalls/Maxim site has loads of **broken link removed**. But just because they are digital, doesn't necessarily mean that they are any easier to use. There are lots of appnotes with code examples that could prove useful, depending on the type of compiler/language and bus you decide to use.
Hiya Tekmin,
Eh mate checkout the microchip site in the analogue setion for active thermistors, they change voltage with temperature and can wired direct to a pic A/D input. Off memory the chip is a AC75 but checking the site will confirm the correct part number.