That depends on the specific website.

For a number of websites, you can find tutorials here.

There's also this webpage with details per country and booking instructions for a number of websites at the bottom.

In many other cases-a real surprise for many a newbee-it is even best/cheaper to simply go to a counter of the railway and have it done there. Again depending on where you are then, they may even be able to arrange it for you in other countries. And NO-one does not in general have to fear/panick about that many monthes ahead.