Buy your Swiss Pass before you go. It's cheaper and you won't have the hassle of buying it from an airport or railway station.
There's not a huge difference between first and standard class. Swiss trains are mostly comfortable wherever you sit and provided that you travel outside of rush hours you will get a seat.
Some routes offer a distinctly better service for first class, e.g. the metre-guage "Brunig Line", where you get a panorama coach in 1st (big windows in the roof).
Switzerland is not in the Euro zone but they will accept Euros. The Swiss make it very easy to spend your money and you'll find loads of cashpoint machines almost everywhere.
While you are there - Remember the old saying "when in Rome do as the Romans do" and don't behave like an American. The Swiss (and most Europeans) are wary of American tourists who insist on being "American" and expect everything to be same as in the USA.
The reputation is of loud, rude people who demand everything instead of politely asking and get upset when they discover that the USA dialect is not the only version of English on the planet.
I have personally witnessed a table of American tourists in a Swiss restaurant in the charming Alpine village of Murren. They ignored the menu and were loudly and unpleasantly demanding "cheeseburger and fries" (not available). They were not drunk, just arrogant. The waiter's patience was commendable. After he had removed them he told me that it was very common behaviour by Americans.
I'm sure that you're not like that. The Swiss are very practical people who really do like things to run on time. Don't be late for your train, bus, aeroplane or any appointment - they won't wait.
Swiss beer is not very good. Ask for Heineken, Kronenburg or any Austrian brew. Please don't drink "Bud" unless it's the real Czech brew (Budvar).
If you have a chance then visit Meiringen (home of the real maringue pudding). Near there is the Reichenbach Falls where Sherlock Holmes was reported as falling to his death during a fight with Professor Moriarty. It's 30 mins walking distance from the town if you can get directions.
It can rain heavily in Switzerland in summer. Probably something to do with the altitude.
The Bernese Oberland is a region that is excellent in summer and winter (of course). There are two main railway lines to get you around. The Bernese Oberland Bahn (BOB) from Interlaken to Lauterbrunnen and Grindelwald and the Wengenalp Bahn which takes you to a place called Kleine Sheidegg. From there you catch the Jungfrau Bahn which takes you inside the Eiger and up to near the top, The Swiss Pass might not automatically cover you for full discount on these lines. You may decide to pay for a "Jungfrau Bahnnen Pass". Check up to be sure what is available to suit you.
If you are a James Bond fan then you must visit the revolving restaurant on Piz Gloria. This was Blofeld's laboratory in the film "On Her Majesties Secret Service". It's accessible by funicular and cable car from the town of Lauterbrunnen in the Bernese Oberland. Time your self for a late lunch there. The Jungfrau Bahnnen Pass is valid for this trip.
Most of the cable cars and local Swiss narrow gauge (metre and 800mm gauge) lines close at or near 6 pm so don't get stranded.
There are several types of fondue, not just cheese. Enjoy.