familysearch.org can be a good place to start as they have extensive records and are a free site.
You could start there with you father's first and last name and his birth year. Your mother's first and last name and her birth year will also be useful in addition to their marriage year and location.
Surnamedb.com another free site has historical information on the Fisk surname, which you may want to look into.
Do you know the county or city in Great Britian where your mother was born and the county and city in Germany where your father was born?
If that is successful, you could enter the same kind of information on your four grandparents and go back another generation to great grandparents.