As many before me said but will say again for the sake of completeness of an answer:
First off provision to have an emergency fund of 6 months living expenses to cover loss of employment, unforeseen medical issues etc.
When that is done you re free to start investing. Do remember that putting all your eggs in one basket enable risks, so diversify your portfolio and diversify even within each investment vehicle.
I would personally stay away from stocks as it's for the most part a bear market right now (and I assume you re not interested day-trading to make any short term return) and most importantly you dont mention any trading experience which means you can get shafted.
Long story short most of these work; mainly for the benefit for their management and people selling them.
Instead, I would go for corporate bonds where you essentially buy the seller(aka the issuing company) and unlike gambling on stocks of the same company, you dont rely on speculation and stock gains to make a profit. As long as the company is standing when the bond matures you get your payment. This allows you to invest with less effort spent on a daily basis to monitor your investments and much better returns(especially if you find opportunities where you can buy bonds from structurally sound companies that have for reasons you deem irrelevant, purchase prices in the secondary market for cents in the dollar) than your other long term "stable options" like German issued bonds or saving accounts that are low in general and more so like in the current situation for German banks.
I would also look into cryptocurrency for the long term as that seems to be past its childhood diseases and its also a good period of time to invest in as even the blue chips of that market are down party due to correction from all time highs and partly due to speculation. As Im more knowledgeable on this than German-locale bonds, a few coins I suggest you look into and decide for yourself would be the obvious ETH & BTC, then a slew of newer ones including but not limited to OmiseGO, Tenx(Pay), Augur and IOTA. Beware though, make sure to understand the basics of security and good practices on this field, as there's no central bank in this sector and if you leave funds in an exchange or your wallet's private key is compromised the money are as good as gone.