There is no one right way to do investing. It always depends on the person.
1. Life goals
Before starting an investment you need the right mindset and you should be aware of your own goals. What do you want in life? Do you like traveling, are you planning to marry in the next few years, get a house and kids? Or do you want to be more independent and maybe switch between different jobs? Maybe a job in an other country? How long do you want to work? How much money do you need every month and how can you save?
Your money should support you in your life goals. Until than it's not bad to save your money in a save bank account with few interest ("Tagesgeld").
2. Get rid of debts and get a risk buffer
Before starting an investment you should get rid of your debts and save some money as a risk buffer. You may save three to six monthly salarys so a broken car, a dismissal or some problems in your household won't be problems for you.
3. How can money support your goals
Okay, you know what you want to achieve and how much money you want to save.
Do you have some financial goals you want to achieve in the next few years (like traveling, a weeding or buying a house)? Than save your money for that.
Do you want to get much money for your rent or to start an ealier retirement (can you save your money for more than ten years)? Than start some investments.
But don't forget, yield is always connected to risk. It's easy to say that you may get 7% yield in the stock market but the stock market is not save. You may get some years 10% years but lose to 50% in other years. Appartements may be a good choice but right now these are very expensive and also very risky for a young man.
Something you should always consider are the costs you have to pay. You want to get a house? Then you have to pay 10-15% for taxes and estate agent. Do you want to get stocks or fonds? Then you have to pay buy fees.
Something that could be a good option for you is a saving plan into a low cost diversified ETF. But only invest that much that you won't sell your shares if you lose half of your money, than you have good chances to get a good yield after at least ten years.
If you want to study some good free German lecture about that, try to read the blog of the German Finanzwesir.