John W Haas Jr.'s Page

I am the oldest of three brothers searching out their German roots in hesse - darmstatt. I have been told that every other person walking down the street has the name Haas or some variation.
Frederick and Theresa Haas came to America as a married couple traveling from Darmstadt by land through France to the port of La Harve. They sailed on the S. S. Rappahannock arriving in New York August 7, 1845. They made their way to Marietta PA and are first noted in the 1860 Census. Tax, census and other records have enabled us to follow the family to the present. I would like advice on how to get at our German ancestors.
Thank you.