I've flown on Lufthansa a lot...always Business Class, which is very nice. I'd say the coach seats on an A340 aren't any better than domestic US airlines, so you'll be pretty packed in. They do announcements in German first, then English. The preflight briefing is done on the screen with an animated program...it's really funny, try not to laugh.
Bring lot's of stuff to do unless you're a sleeper...I can't sleep on planes. Walkman, Gameboy, a book...whatever, and they will show two movies, so get the headsets. If your a typical teenage male and need to eat a lot you should bring your own food or you could get pretty hungry between the two meals you get.
Frankfurt is pretty cool...you'll connect there. It's a huge airport and great for watching planes...I've noticed a couple of nice observation decks but I always head for the lounges to drink beer. The airport isn't really fancy but efficient in a German sort of way...you'll see what I mean. There is a long underground walkway between domestic and international, just like UA at ORD. Also, very interesting people watching as you see all kinds of dress from all over the world.
You won't impress the Germans by running around being loud kids...have fun but be polite and you'll be treated with the minimum of kindness in return. I've always been treated okay but have always got the feeling they don't really like Americans. Other parts of Europe I've been to are great...Poland, Norway, and Denmark, absolutly ROCK.