No reservation needed
If you wanna reserve it´s quite easy via oebb.at and the seat only option (3€ per seat)
If you travel with your family you can reserve up to 5 seats (2adults and 3 childs ) for 8€ 2nd class or 10,60€ in 1st class via bahn.de
Have a look in this post
From 12/6 prices on bahn go up with 50 cts for the RESrv. (or 1€ for family)
There are a few through trains and more with a connection somewhere-REServ is optional, (and not at all possible even on REGional trains), but these trains tend to be pretty full at most times so avoid standing or sometimes not even be allowed boarding by doing it. Do it via bahn or oebb-as seewulf already explains.
If you are in a too full train-get off in LInz-from there mutliple nice RailJet trains per hour onward.