You can generally drink water from the tap in Germany, as tap water is subject to strict controls. There are exceptions, and these are indicated with the sign "Kein Trinkwasser" ("not drinking water"). You cannot drink the tap water in trains, for example.
These rules apply for alcohol:
You can buy alcohol in nearly all supermarkets, restaurants and cafés. You must be at least 16 years of age, and for some alcoholic beverages even 18 years of age. It is best to carry your ID card with you. In some places you are not permitted and are discouraged from drinking alcohol, for example on public transport. You are not allowed to drive a car or ride a bike if you have drunk alcohol.
Not everyone in Germany drinks alcohol. People will not give you strange looks if you would rather drink juice, a fizzy drink or water when you are out in the evening.