Before you can search in Germany, or any of the former German territories you need to know what town or village they were from.
Possible sources for this information are ship manifests, naturalization documents, obituaries, and possibly family documents or letters. If old family letters are still in their original envelopes, the town name in the postmark may be a clue. Don’t discount family stories for clues, either.
Death certificates may have that information, but usually just the name of the country.
Former German or Austrian territories will have the location name in German and the national language of the country today. Sometimes the two names are similar, sometimes quite different, for example, Lednice in Czech and Eisgrub in German). There are gazetteers for both Germany and Austria that can help.
Church records can be in German or the national language depending on the ethnic make-up of the area. Older records are frequently in Latin. More recent records tend to be in the national language.
Knowing what religion your family was can be helpful. Northern Germany was frequently Lutheran (or Evangelical), southern Germany, Austria, and many of the former territories were Catholic, but this is variable. Jewish communities were in many areas, especially the area formerly known as Galicia.
Many Catholic and Lutheran records exist from the period of the Thirty Years War (1618-48). Records are sparse before this.
Jewish records can be sparse, but some do exist.