To add to what Balso wrote, the Swiss Pass covers ALL transport in Switzerland, trains, boat, trams, rural and city buses. You can see this on the map Balso posted. The only thing not covered are some mountain railways and cable cars which count as "tourist attractions" and not public transport.
You don't have to "do" anything to "use" a Swiss Pass, just get on the train/bus/boat, pick an empty seat and sit down. Yes. it is that simple. Somebody will come round during the trip and check your ticket - sometimes, other times they only have time to check part of the train.
"Is it free seating" - no extra costs for seats ☺. No reserved seats, sit where you like (except seats somebody is already sitting in).
"how about the deck between first class and 2nd class" - not sure what you are asking. Some trains are double-deck, most are single deck. You can sit in either deck. Of you have a 2nd class pass you must sit in 2nd class.
"Can i use it from zurich airport to zurich HB" - yes, all transport means all transport.