Here is good information on how to make reservations.

If I am correctly informed the reservations for Portugal have to be made in Portugal and most of the Spanish reservation also have to be made in Spain.

In the good old times you could book in London or Amsterdam a ticket till Russia and even further but with the digitalisation all changed and each Railcompany use a own system :/
Because of that no regular ticket office in the UK can reserve trains outside of the UK (few specialised ticket agency´s still can do it )
Amsterdam
they are able to reserve more but how much is quite difficult to say its a try and error :/


Spain and Portugal made it quite hard to get reservations from abroad.

The staff -for INTern in Ams CS (yes that is the main station-like Flinders in Mel) will NOT do ONLY REServ for trains other as those starting in NL-as they are overworked, think they are underpaid and in main season the Qs for normal tickets are so giant+huge that there is no time for such fringes. Plus that on any order there is a service charge of 7,50 extra=more as a normal REServ cost. If you were USA you would also shiver at the ´time lost´ for that waiting.

Look mate, this is Not VicRail or the Ghan with just 1 train/week or so-the routes you mention have trains about ev hour so if you simply have the slightest flexibilty to go maybe 1 hr earlier or later you will get a seat a day or 2 before-done locally. Which also much lessens the need to preplan everything weeks/monthes ahead-in general not a very strong thing for OZzies from downyundah.

Reservations for Portugal can only be done locally. Reservations for Spain are available in some more places but still quite limited (not in Amsterdam or London) because Spanish railways make it hard for others to book. I just posted an overview here.

Italian day trains can easily be booked online via ÖBB, see these steps.

so basically just wait will I get to the destination country and hope for the best?

That depends on where and with what type of train you plan to travel. If you share your travel plans here you can get advice on if there are reservation that you can make in advance. 

I’m going to a lot of places!

June
Amsterdam, Germany, Belgium, Austria, Budapest 

July

france, Spain, Portugal, Prague, Croatia

august

Greece, Italy, Nice, Switzerland 

That seems really nice. 

Are you planning to make as many reservations as possible in advance or only to make reservations for the trains where reservations are mandatory, like high speed trains (not in Germany) and night trains?

There are many train where no reservation is required and in some countries reservations are rare. The Eurail planner is not always up to date on the need of a mandatory reservation, or even the possibility to make reservations, so the advice is to check the website of the railway company that runs the train to get the accurate information. 

As an example, in Switzerland, only a few scenic tourist trains require reservations. On the routes for these trains there are also regional trains without mandatory reservation.

I will be Interrailing with my daughter this summer in Germany, the Czech Republic and Switzerland. The only reservations that we will have is the night train between Sweden and Germany.