The only bus service from the airport into the city centre is here:
http://www.sadem.it/media/6355/000268.pdf
However your hotel is so close to the airport that the first stop on the bus route (just 3 minutes into the journey) is actually beyond the hotel.
As the hotel website tells you, it only takes 3 minutes in a taxi from the airport
http://hotelniagaratorino.it/ita/index.html
so that would definitely be your easiest option.
The fare all the way into the city centre (16km) is estimated as between €30 and €50
http://www.aeroportoditorino.it/en/tomove/parking-transport/by-taxi
so the fare to your hotel should be well under €10. (The bus fare to the nearest stop is, which would still leave you quite a walk, is €2.10 per person if you buy it at the airport ticket shop or €3.10 per person if you buy it on the bus).
(Note: The relevant train service doesn't run late enough for your needs but it might be worth considering for your return journey to the airport. Caselle station is 260m from your hotel, with Caselle Aeroporto station the next stop on the line. The railway line also provides your best way to and from the city centre:
http://www.sfmtorino.it/le-linee-sfm/sfma-torino-aereoporto-ceres/ )