Enquiring about "family trees" and citizenship by decent

You can only get German citizenship by descent (you don't need to be decent for this  ) if at least one of your parents was German citizen at the time of your birth.
Researching further back is therefore only required if the citizenship of your parents is in doubt. If you want to do that, you should contact and visit the family registries (and before WWII sometimes also church registries) of the places where your ancestors lived. There is no central place for keeping such documents, and all of them are available on paper only (meaning extended travel and detective work is needed to find them) - IF the records were not destroyed in the war or through some other misfortune.
There are companies offering this as a service, but it won't come cheap!