well culture and religion are woven together very tightly in tibet, nepal, and india,so you'll definitely want to try and learn what you can so that you'll understand the things you're visiting, as well as the significance they have for the people of these countries.

learn their cultural norms as best as you can so that you can blend in a bit better than the average traveller.

read up on high altitude sickness, things you might be prone to since you'll be in a completely different environment, expect to get sick

just prepare yourself as much as possible, but when you arrive, no matter how prepared you are, bet on the fact that you'll still be in for a huge culture shock