Planning
If you have a destination in mind, there are many good sites for planning how to get there.
The Man in Seat 61 offers detailed advice on travelling by train and Google Maps and Rome2rio shows how to get from somewhere to anywhere, such as Rome to Rio... For paper-based products there is https://www.europeanrailtimetable.eu/
I found a problem with most existing sites - they assume you know where you want to travel and then try and get you there as quickly or cheaply as possible. I made Trip-Hop for people like me who want to slow down and explore Europe one hop at a time. I also found out that sometimes coaches are not just cheaper but quicker and a better ride. Anyway, have a look and let me know what you think...
Buying tickets
The Trainline and Rail Europe are both reliable sites for buying tickets that span rail networks and I would happily use either of them. The DB App is also a sound way to look for and buy tickets.
Things I've learned
travel light (or at least, make sure I know where I can stow my luggage)
Over planning can be stressful, especially if it involves tight connection times. Try to slow down and be a bit more spontaneous.
take water (in a reusable water bottle), a snack (in case things don't go quite as planned), hat and alcohol gel
when you travel alone, people talk to you
don't panic