What is the best way to travel in Europe?

I’m spending the summer in Europe and I’m planning on traveling quite a bit. I want to go and visit Greece (Athens), Switzerland (Geneva), France (Paris and Nice), and Spain (Barcelona and Valencia and Madrid). I’m looking into rail passes, but I’m still not quite sure how they work. Is there one site that’s better than all the others, or the best deal? I want to be able to plan ahead but it’s kind of scary since I’ve never done this before!