Your question is about as broad as it gets. With no idea where to begin, you might want to try what I've been doing. Browsing the forums, and keying in on thread titles that look interesting. Tons of great information, ideas, tips, etc... have already been written.
Maybe after you have looked around for a while, and narrowed down what areas/cities you want to visit, and what you want to see and do, you'll have some more pointed questions.
I'm just now completing the planning, reservations, tickets, and other travel arrangements for a two week air/rail/rental car trip through Southern Germany/Austria in mid September. (Octoberfest) 70% of the information I've needed came from reading these forums. The rest came from Rick Steves Germany book, and internet search.
This is the first post I've written on this forum, since all my questions were answered without asking :)
Good Luck, Planning should be fun!