Just to add...  There are frequent trains from Frankfurt Hauptbahnof to Darmstadt.  It's about a 25-minute ride depending upon if you take an express or a local train.  (You can also catch a train from the airport.)  There is also a bus that runs from the airport to the Maritim hotel which is right next to the train station.  It costs something like €7.00 each way.  Once you get to the train station, it's about a 30-minute walk or a 5-minute taxi ride.  I don't recall a tram going near Sharks, but I'm pretty certain you can catch a local bus from the train station if you don't want to take a cab.  

For the trains, you can use the www.bahn.de site.  There is an English option which makes things very simple.  And, yes... German trains run on time!