Hi Lynne,
I just did this in May with my wife. We flew into Frankfurt, rented (from Enterprise) and drove all through the Rhine, Bavaria, into Salzburg and then back to Frankfurt. I went to AAA and got an International Drivers Permit (it's just a booklet that tells a police officer how to find the info on your license in their language, and not really a license at all). I needed both it, my Washington State DL and a major credit card to rent my car.
The vignette is something you can buy from any gas station (there will be huge signs on the autobahn as you near the exit for said gas station) for roughly 10 euro. Fix it to your windshield and don't sweat the worry about getting a fine.
Road signs were fairly easy to figure out, but I also rented a GPS unit that really made it pretty simple. I was told where to turn well ahead of time and also alerted that I was speeding with an audible beep. This is something you will need to pay attention to, because speeds change all the time, both on the autobahn and the side roads.
All in all, I found driving in both countries very easy to do. Just remember that when you are on the autobahn, never pass on the right and stay out of the left lane (unless you are passing someone). Look in your rearview often to know when someone is flying up behind you and you'll be good to go!
Enjoy your trip!
RF