In Germany, there are two types of post offices: the normal “Post” and the so-called post agency (Postagentur). Post agencies are newsagents or stationary shops with a post office counter inside. They offer most of the usual services. You can identify both types by the yellow “Post” sign outside.
If you need a postcode in Germany, you can find it online.
How to address a letter in Germany (pdf-file)
When moving, you may want the post to redirect your mail (Nachsendeauftrag) or store it for you for a certain period of time. There are different services available (e.g. 6 months redirection: around 15 Euros).