Alpine route

First time traveller to Japan. I am getting jr pass.
How should I plan my trip to alpine from Tokyo.
Should I purchase tickets in advancd

How should I plan my trip to alpine from Tokyo.

Decide on the overall itinerary. Here are some options:
https://www.japan-guide.com/e/e7552.html

Should I purchase tickets in advancd

I always make seat reservations for long-distance trains in advance for peace of mind, but if you do not mind the possibility of not finding a nice seat, it is also possible to travel without seat reservations.

As for the Alpine Route itself, it is possible to make reservations only for the first trolley bus from the Nagano side (Ogizawa) or the first cablecar on the Toyama side (Tateyama Station).

But with a advance reservation comes inflexibility, which is problematic because you want to visit the route while the weather is good. Here is the reservation website:
https://tateyama-kurobe.alpen-route.com/ticket/en/?departure=ogizawa

As you can see, it is easier to make reservations on the Nagano side (Ogizawa).

More planning details can be found here:
https://www.japan-guide.com/e/e7551.html