Hi Margarida,

some more details would be good.
Which route/country/trains do you plan to take?
Usually you can make the reservations in the ticket office at the stations. For some countries it is also possible online. Interrail offers a service, but it costs 2 euro for each reservation extra! 

For some trains a reservation is not obligatory but possible. For some trains reservations are not possible (regional trains in almost all countries). In some countries reservations make no sense (Switzerland, Netherlands, Belgium e.g.). 

@MartinM  I answered here already in a Facebook group it were several mainly in Poland / Slovakia/ Czech republic 

We look for a way to reserve Krakow - Bratislava Nighttrain online except PolRail

Suppose that is online not possible without extra fees.
But why not doing it in Poland at the station? Shouldn’t be a problem to get a reservation 1-2 days before travelling. In worst case that it is sold out, there are still some other ways with leaving a bit earlier and take a bus to Cieszyn, walk over the border to Cesky Tesin and from there take a train to Ostrava and continue without a reservation to Bratislava on the same train.

Also possible: buy reservation for Krakow-Chalupki online on intercity.pl and later buy Chalupki-Bratislava at the ticket office.

Hi was able of doing some reservations by going to the sites of the train companies, but I haven’t been able of making a seat reservation on trenitalia, do you know how I can make one? Thank you

Go to the station in Italy and buy it there. Should also be possible at stations in Switzerland and Germany. In case you are in these countries before Italy.