If you are employed, you must already be contributing to public retirement insurance. It is debited directly from your salary, and you get it when you retire (70% of the net income). If you have noticed, Germany has already increased the retirement age from 65 to 67, and there are chances that it may further increase until you retire.
Investing in voluntary private pension plans can be a good option. It would work as an add-on to the existing public pension plan assuring you to have a good life after retirement. You can get them covered from any part of the World.
With these plans, you can start getting your pension from the age of 60.