It depends on where you’re going! Have a look here. The info on Seat61 is a great summary of where to make reservations for all the different operators.

https://www.seat61.com/interrail-and-eurail-reservations.htm

I guess you're talking about Eurostar, right ? (according to your post history)

Eurostar has a passholder quota and trains can sell out days or weeks in advance. After that obstacle it will be a lot easier.

As I said in my other post there are good ferries as alternatives. Please give us more details, incl. route and travel date. We'll gladly help !