As you have chosen PIC already - that's a good start.
Probably something entry level with internal oscillator - 16F628A or ...
Easiest to start with MPLAB as your IDE/compiler (free).
Next is the type of sensor - I2C, or one-wire ? Direct reading analog (thermocouple)?
Then choose the display medium (LCD, LED, meter etc)
Once you have these - you will have learned a lot from the research into each option.
There are hundreds of code examples fr complete clock / thermo projects
You need to 'skin your knuckles' a bit - trying - and failing - then you'll know what questions to ask here next!
Get your core components together - blink a LED to make sure your tools are all in order. If it all seems too hard, perhaps try Arduino. There are a lot more entry level beginners to share code samples.
Good luck.