As a US citizen, you owe income tax on all your world-wide income, even if you are living outside the US. If you meet the 330 day rule, you can exclude your foreign earned income (income earned from working, from providing a service or selling goods), but you must still file a tax return and pay US tax on any unearned income (IRA withdrawals, investment income, etc.). If you also pay foreign income tax you can claim an offsetting deduction or credit on your US return.
You are entitled to file jointly if you wish, but that might subject your spouse to US tax. To file as married filing separately without a tax number, you will probably need to use a fake tax number to get Turbotax to print, then cover it with white-out, write "NRA" in the space for the tax number, and sign and mail your return. You will want to use a mailing service with tracking and proof of delivery. You can't e-file if your spouse does not have a tax number.
Also, you will need a US bank account to receive any refund (or else the IRS will mail a paper check to Germany). You will also need a credit card with a US address to pay the Turbotax fees, since Turbotax is technically not allowed to be used overseas. Or, you could buy a copy on CD from a US vendor, but they might not ship overseas either. You might have to get a US friend to buy a copy for you and mail it to you.
