Hi Sahil -
rockoyster has given you the answers to most of your questions.
I'll add a couple of comments.
Getting Started -
On MySwissAlps, first time visitors to Switzerland can learn a lot from the introductory pages under "Getting Started" on the home page.
Specific Cities and Regions -
You specifically asked about Montreux and Grindelwald.
In general, check for the names of cities by using the search tool (top right of every page), or by using the links under "Where To Go." Under the Towns and Cities Link, click on Explore Swiss Towns and scroll down under "Browse all Towns in Switzerland." Many cities are on a list , so that you may not need to use the comprehensive search tool available there.
Both Grindelwald and Montreux are there, as well as Interlaken ( where you will change trains for Grindelwald) and Luzern (which is the German spelling of Lucerne (French spelling) that you will see on signs in the German-speaking regions).
In general, except for such special panoramic trains, you do not need reservations on Swiss trains within Switzerland.
And, as noted by rockoyster, and is explained further on the main Golden Pass page, you actually do not need reservations to travel on many of the trains on the Golden Pass Route.
All details are listed on the main link:
with the specific details at the more detailed link provided by rockoyster.
However, the most scenic part, with optional different types of special panoramic or historic cars , is the last west bound section from Zweisimmen to Montreux, especially after Montbovon. You have the option to reserve in those trains with special cars, and should do so, if you wish to sit in them. They fill up early, so they require reservations. It is not necessary ride the special trains, but they add to the experience. That is especially true of the trains in which you can sit at the very front of the first car.
If you use the time table as noted by rockoyster and click on the line (which is a link) for an individual journey after you enter your start and destination (use Zweisimmen and Montreux) and date and time, it will expand and tell you whether the train is Panoramic or other wise special.