It is probably best to start with a few general sites.
Resources for German Genealogy on the Internet:www.germanroots.com
Introduction and Links to Resources:http://www.archives.gov/genealogy/naturalization/
When posting a message it is best to give further details such as ages, birth and death dates. If not known, then an approximate year would be good. This will especially be true for a common surname of Rogers. You might find in your research that Koehn will be spelled Cohen or Kohn.
Best of luck!