As above, no need to buy the tickets in advance. Not would I want to, as you have 2 choices for your route, and you may want to wait til you arrive and see how fatigued you are before you decide. Also, you may or may not want a Swiss Pass for the rest of your trip.
The faster route via Bern takes 2h 40 minutes and costs 66CHF per person in 2d class. There is a minimum of 2 changes along the way, at Bern and Interlaken Öst ( avoiding a change at Zurich main station and Spiez). Such trains depart the airport station once an hour, at XX:45. If you don’t care to wait, there are lots of intermediate trains with a change at Zurich main station and/or Spiez. I probably would choose to wait rather than change at Zurich, but Spiez wouldn’t be as bad. You could use the wait time to find a snack to carry on board if you like.
The slower, more scenic route goes visa Luzern and over the Brunig Pass. This trip takes an hour longer but costs less (52 CHF) because it is regional trains rather than Intercity. I believe the views are on the left side on that section, but maybe Nigel can confirm. Brienzersee (Lake Brienz) is definitely on the left, but I am not sure about the waterfall near the pass.
The “scenic route” has a minimum of 2 changes, at Luzern and Interlaken Öst ( you will always change there to get to Lauterbrunnen). They may split the train, with part going to Lauterbrunnen and part going to Grindelwald, so be sure you board the right section ( there will be signs).
The most convenient trains with 2 changes on this route depart at XX:38. There is another direct train to Luzern at XX:15, but then you will have a longer wait for the train to Interlaken, as they only depart Luzern hourly ( at XX:06). But maybe you would rather spend wait time at Luzern rather than the airport —-with 40-45 minutes you have time to walk outside and look around a bit. All other trains leaving the airport will have a change at Zurich, which as a newbie I would avoid—the station has several levels and can be confusing.
The Intercity trains on the Bern route have food on board but I don’t know if it is an actual café car with seating. On the Luzern scenic route, one section shows a food symbol—-between Luzern and Interlaken. Again, I do not know if that is a dedicated car with seating, or a counter with items to take back to your seat. We have only ridden in 2d class on Swiss trains, and never look for food on the train. Sometimes we buy snacks at a station if there is time.
You can see the routes and schedules, as well as the amenities on each train segment, on SBB.ch. Or use rail.ch to go directly to English. Put in your start and end points, and a date and time. Choosea date in the net week or so to get an idea, if your travel date is farther in the future. The schedules do not change much at all. Once it opens a page with 4-6 options, look at the number of changes, price, etc. ( The price shown is half the regular price, as it assumes you have a HalfFare card). Touch the black line showing the number of stops for a particular train to have it drop down a list of the individual trains on the route and the specific details ( food on board, etc) as well as the expected load level ( that is only useful close to your actual date).