I pay taxes in the country in which I reside (Germany). I have no assets in the U.S. What form must I file?
There is a Tax Guide for U.S. Citizens and Resident Aliens Abroad available that deals specifically with the issues.
Personally I would suggest contacting an accountant that is familiar with filing such returns.
All the same forms any other American must file. Read instructions carefully, including the tax guide Karlson linked to.
However, since you don't live in the US, you are very likely to be subject to FBAR and FATCA reporting. While FATCA forms are part of the tax return - FBAR are not, filed to a different government agency, at a different time, and carry very heavy penalties if you forget or miss a deadline.
Another thing to keep in mind - if you invest in mutual funds in Germany you're probably subject to PFIC rules (a specially devised rule to discourage Americans from investing outside of the US through very heavy taxation and draconian penalties).
If you have a German pension/tax-deferred account, remember that this is taxable in the US and is not included in FEIE. I.e.: while you're saving tax-deferred or tax-free in Germany, Uncle Sam will tax you on that without any regard to the German tax-deferral rules.
If you own a significant part of a German company or business - beware of Subpart F income and forms.
If you own or are a beneficiary of a German trust - yet another can of worms. Self-sponsored pensions fall into this category many times (together with PFIC).
Bottom line - get rid of your American passport. American tax laws are devised to make it nearly impossible for a US citizen to live outside of the country without becoming a criminal.
If you still want to try - don't even think of filling your tax return on your own. Get a US-licensed CPA/EA who is also proficient in German financial terms and German-US tax treaty to do it for you. The tax treaty may include some provisions which may alleviate the pain somewhat. Yes, it will cost you about $1000 a year, at least. That's the cost of maintaining your citizenship.
In this answer, you can substitute "Germany" with any other foreign country. As long as you don't live in the US - you're in trouble, tax-wise.
Some countries reached an agreement with the US re the pensions (Canada, most prominently), but most haven't.