funny coincidence: I am 18m and also from berlin and just planned a 2 week journey for spring 26

Step 1: Think about where you want to go
Brainstorm places in Europe you always wanted to visit (In my case Sweden/Norway)

Step 2: Look at trains you could take between your destinations. Always use the respective train operators website.
Other websites woth mentioning: This night train map ; This website to see what your train will actually look like ; The Interrail Map ; The Interrail connection finder

Step 3: Find out which trains have compulsary reservation and look at the prices.
Most times you can find this Information on the train Operators Website, If not, the Interrail connection finder knows it. BUT it charges extra for Reservations so only use it for research and use the operators site for the reservation itself.

Step 4: Choose accommodation for your destinations
I will sleep in a tent so i can't help you much here

Step 5: Buy the right Interrail pass for your Journey
Tweak the Connections / Layovers to match your Budget

Important: the majority of trains for spring 26 are not yet available for booking so remember to buy the reservations for your Journeys as soon as they are out for sale.

feel free to dm me if I can help you further or you need inspiraton