On Switzerlant]d it is never necessary to buy train tickets ahead of time. Trains are mass transit.
So you can just go to the ticket office at Zurich airport, buy the half fare card an your tickets, and go.
However, there are some good deals to be had when biying in advance.
The SBB now sells supersaver tickets, and the so called « saver day pass » (SDP) which is a pass allowing unlimited travel om all public transit for a whole day. For someone arriving at ZRH this may be a good deal.
So just go to www.sbb.ch/en and see if that is available for your day of travel. The SDP is cheaper with the Half Fare Card, but you do not need it yet when booking, only when travelling. So buying tickets or a SDP with HFC reduction now, with the intention to buy one once in ZRH is fine.