If you are new at this and are very concerned about how to travel and buy tickets, I would strongly suggest you call this company. It is based in the US (New York) and they will sell you almost any ticket for train travel in Europe. They will also include some brochures with explanation.
It will cost you more than going on separate websites or buying directly in the country but it is worth it.
I have no financial interest in this company but have used them (and recommended them to others) on a couple of occasions.
This is the link of how to contact them:

https://www.raileurope.com/about-us/contact-us2.html