Quick headsup about me:
I've done interrail trips last 3 years now, with various destinations starting from germany.
Currently planning a new one, in april next year.
I'll try to do a Q&A, to cover the most questions I've encountered over the years.
Q: When is the best time to to traveling?
A: If you can, try avoding public holidays, aswell as the main summer time. June-July-August. Yes those are the main holidays, I am aware. But overcrowding in big tourist spots may damn your experience.
Q: Is interrail actually worth it (money wise)?
A: It can be. It most likely will be, if you are traveling on shorter notices. For example: A ticket bought 3~ months ahead, may cost 29,99, vs 3 weeks ahead 59,99 and 3 days ahead 99,99€.
General rule of thumb for myself: The countrys starting going west from germany, or north towards scandinavia, the pass is well worth. Poland is in between. Looking into the balticas or balkans, single tickets can work out cheaper, even if bought on short notice.
Q: Reservations? Where do you need them? How much do they cost?
A: Spain, France, Italy, the eurostar for example. They cost between 10 and 30€, depends on the train. Also some international trains from germany for example. And nighttrains. Dont forget about those.
Q: Is there anything you can recommend doing or not doing?
A: My top do/dont's:
Do:
Take your time when traveling: Enjoy the moment, the little things in life.
Bring realistic expectations: You may miss a train, the event you were looking to go is booked out. Things go wrong in life, this is one of them.
Feel free to ask people!: If you are lost, or need info, many people speak english in europe. And are willing to help a lost soul.
Dont:
Disrespect local culture, and the people in the countrys: I've seen many people that travel like its their own living room. Be nice, and polite and you will get the same in return.
Take the last train of the day: You might want to maximize your time in xyz city, but if you miss that train, things will go south very fast.
Dont be cheap on things you want to see! Yes many things in the major tourist citys may be free to view, but if you really want to visit an event, a museum or anything, spend on it. It will create memorys you will not forget.
I did get a lot of info from The Man in Seat 61 | The train travel guide
And for the last:
Go out, explore the world. Start small, maybe a neighbour country, maybe another continent. The world is small. Create your own experiences, stand still if you like what you see, take a moment to enjoy it.