Heavily depends on the route. If you want precise advice give us more info :)
Either you can make reservations for free/a few € (through the railway company) or you can’t : this is the case for suburban/regional trains everywhere. For example trains in Belgium, the Netherlands + most Swiss trains do not have reservations.
It works in a first come first served basis. It is very rare that you have to stand though (except at rush hour in cities), there are loads of seats and nobody will have a reservation. You are always allowed to board so it’s unnecessary to arrive hours in advance : 10-15 min is enough.
As a general tip : go to train ends, people are lazy and board in the middle so you’ll guaranteed find more seats there.
How to book depends on the train. Have a look here:
https://www.seat61.com/interrail-and-eurail-reservations.htm
If that doesn't help, then please mention route, date and departure time of the trains you'd like to book.