You can reserve seats at a ticket office, via phone or via a reservation service like Interrails own  and some others Which Route are you looking for?

Hi Seewulf ! 

Thanks for your answer. 

I’m looking for the train from Paris to Milano

I made a reservation but they said I have to pay a complement but how can I pay it ? How can I know my seat number ? 

adding the train to your Journey on the RAilplanner is not a reservation :)  Thorugh the booking process on the website of Interrail they guide you through several steps one of them the payment step the last one is to get the tickets shiped by mail (in case of paperreservations) or send by e-mail (in case of E - Reservations) thw train Paris - Milano have limited seats for Railpass holders with a reservation price of usually 25 or 32€per seat Yeah SNCF charge quite high fares and vanished their own Interrail reservation option on their website :/

Addition by moderator: Your inbound/outbound journeys are not on top of your total travel days but part of your total travel days. When travelling with an Interrail Global Pass (as a resident of Europe), you can use one travel day to leave your country of residence (outbound journey) and one travel day to return (inbound journey). You can use these two journeys in your home country when using travel days at any point in your Pass’s validity.