As others have said you don't really need a car in Hamburg... in fact in some parts of Hamburg you really don't want one because you'll never find anywhere to park it!

Public transport in Hamburg is excellent - as is the norm in Germany it's an integrated system, i.e., there are different operators but they have a joint ticketing system; you just buy one ticket at the start of your journey and it covers the entire journey regardless of what modes of transport you use (including river ferries!). You can also buy day passes.

The web site for the Hamburg public transport system is at www.hvv.de (the "English" button is near the bottom of the page)

Have a good trip over - and do come back and ask more questions when you've arrived!