Hi all, my first post here!

I have an old Trek 810 ST 21 Speed
It's well worn but I still love it, so even though it's probably not worth the money I still want to fix it up.

Can anyone tell me the exact specs of the parts I need to order if I want to replace the:
- Front chain rings (whole crankset?)
- Front derailleur
- Rear cassette (although I suspect it's a freewheel)
- Rear derailleur
- Chain

Are they all super specific or can I use generic parts? Which ones?
Thanks!