72625 posts
Hi fayefaye25 and welcome to MySwissAlps!
So you can buy a ticket to Interlaken from Paris here(and make reservations if you wish).
As far as rail passes are concerned, you can buy a Eurail Select Pass for France and Switzerland if you wish (and you are traveling by train a lot in France as well).
If you are just planning on buying a pass for Switzerland and you are traveling daily for the 7/8 days, I would recommend the 8 day Swiss Travel Pass – it will cover almost all trains and boats etc. in Switzerland. The exceptions are some trips up into the mountains, like Jungfraujoch where you get a 25% discount on the train ticket.
If you like the idea of a Swiss Travel Pass (print at home and almost never have to buy any tickets while in Switzerland, except my comment above) then you just need to buy a ticket from Paris to Basel (1st link) and the Swiss Travel Pass covers the rest.
Don’t worry about making reservations for trains within Switzerland – it really isn’t ever done except on panoramic trains.
Regards,
Lucas