I'd ask the customers you do have why they downloaded the app, what they were looking for when they found it, how they heard about it, what they like/don't like about it, and how they think you could reach more people. What pain are you solving, who experiences it the most, and how do you reach those people?? Answer those questions and you're on your way.
Every business is different and without knowing more about your app, there is no way to give you useful information that will make you money.
You need to setup a Pay Per Click (PPC) campaign, targeting areas like Germany.
You'll need to build a good keywords list and have integration with Analytics to maximize the efficiency of your campaign.
If you have any detailed questions feel free to contact me.