You don’t have to plan out every second but it is a good idea to hav a plan. Lucky for you, you have lots of time for that. My best suggestion would be grab a Lonely Planet Western Europe book, it will end up being your traveling bible and best friend. It will be a good source for hostels, hotels, museums, attractions, coffee shops, bars, etc, etc. A rail pass is a great idea, travel insurance is super important too and often over looked, a ISIC card (international student ID card) will help get you some discounts too. Most importantly your sense of adventure and an open mind. Planning every detail isn’t important but it’s nice to be one step ahead when you can. May isn’t too early at all, sounds about perfect actually, any earlier and you might not get the best deal. You might think abotu at least booking your first couple nights when you land so at least you have someoen to go and can get your bearings once you arrive. Hope this helped a little. Feel free to ask more questions anytime. My email is klucchesi@statravel.com.