Couples planning to get married in Germany should get started with the legal formalities as soon as possible. Six months early is not too soon. Things can usually be disposed of in far less time than that, but a number of legal issues, particularly previous marriages, can create a hassle.
The first places to check when you decide to get married are your local embassy or consulate and the local magistrate's office (Standesamt). Ask at these places what documentation is needed. You will most likely need a birth certificate, a passport stamped with proof of residence (Aufenthaltserlaubnis) and proof that a previous marriage has been permanently terminated. You will also need a health certificate, a statement of parental consent (if a party is under 18) and possibly other documents. If they aren't in German these documents must be accompanied by a certified translation.