City Maps, Hamburg
One of the greatest online map resources EVER for Germany is
It has a great search engine that allows you to pinpoint your destination's exact location marked by a big red circle. You can choose the size of the surrounding area you want shown around your destination, and the map is very clearly marked with highway and subway station information as well, to help you figure out the neighborhood immediately surrounding where you want to go. Great for when you're navigating your way around on foot, or even to familiarize your taxi driver with an obscure location you making your way to.
The easiest way to ensure you get a correct map is to make sure you type a complete destination address into the search engine, zip code included if possible. The maps contain street names, train stops, major tourist landmarks, and highways into and out of the city. There are scores of other maps online for navigating Hamburg, but I find the ones on this site particularly useful because there is a strong attention to detail. This is more helpful for people who are completely new to Germany as it includes smaller streets and a lot of graphics.
Stadtplandienst also a pay service to download maps onto your phone or electronic organizer, on the left hand side of the homepage.
Once you're in the system, you can study the U-Bahn/S-Bahn map to figure out the best route to your destination. The brightly colored lines converge on a few stations in the central city, particularly Jungfernstieg and the Hauptbahnhof.