Where can I find my former levels to strengthen them?
I'm in level 9 without strengthening my skills. Where can I find my former levels to strengthen them?
I can't find German stories, who can help me find the German Stories?
You acquire a new level when you pass certain xp levels. No way back. You can only strengthen the lessons you passed on your way up.
There are no German stories (yet). They are being tested in the lab for Spanish and Portuguese only. Maybe they will be expanded to other languages, maybe they vanish, that's up to the Duolingo team. So all you can do is cross your fingers that Duo marks them as a success.