Paris to lauterbrunnen by train

TGV trains are fast and frequent from Paris Gare de Lyon to Basel; frequent trains from Basel to Interlaken Ost, the narrow gauge train up to Lauterbrunnen.

Just 2 changes, but end to end it will take some time.

Easy. I've done this many times.

One TGV (high speed train) every 2 hours Paris Gare de Lyon via Basel to Zürich. The timetable leaflet is here: http://tgv-lyria.com/main/FCK/File/site_en/Pdf/horaires/HORAIRES14-WEB-SW-PARIS-ZURICH.pdf

At Basel change to a Swiss intercity to Interlaken (2 trains per hour). One train per day divides at Basel and half goes to Interlaken. In this case you don't even have to change at Basel.
At Interlaken change to a local to Lauterbrunnen (2 trains per hour).

Typical price CHF 219. For end-to-end times, prices and ticket purchase see http://sbb.ch

Your ticket will include a reserved seat on the TGV Paris --> Basel. Basel --> Interlaken and Interlaken --> Lauterbrunnen no booking, just jump on the next train.

If you can book ahead by 30 days, there are TGV tickets available to Basel for as low as 49 to 59 euro. Add in the 65 chf Basel-Lauterbrunnen leg and the ticket should be bit more than 100 euro.

How to Look Up Train Schedules Online gives you the DB train schedule link and tips for using it. You can book ahead for the reserved TGV leg through Rail Europe or CapitaineTrain, which both have some advance discounts. If you are considering a Swiss Pass, then that coverage starts at the border, such as at Basel.