That really depend where you are travelling and when.

If you describe your travel plans you will get better advice.

The common risk is to have to stand -part of the way.

Newbees often think that any vacant seat is free: there are screens above/on the side that will indicate when it is taken.

Thanks, so as a newbie can I just check then that if no reservation is required and the train is full, I can still get on it albeit I have to stand?

For trains in Germany yes, Worst case you stand. Maybe sit on the toilet .. lol.

You can check for German trains or for Swiss trains on the app of the company there (DB Navigator for Germany and SBB App for Switzerland) what their predicted occupancy of the train.