You need German if you live in Germany. Some people may be able to help you in English. But everybody speaks German.
If you speak German, you can:
You can do all the little everyday things and you are in control. Speaking the local language really smooths things a lot.
If you want to study at a German university, you need a TestDaF or DSH certificate. Different universities and fields of study require different test results. If you know, which university and course you want to sign up for, you can check which certificate will be accepted and which result you need.
To find a job in Germany the B2 exam is most suitable. In some cases a B1 certificate can be enough.
For a settlement permit or to acquire German citizenship you need a B1 certificate and for a visa to join your spouse (Familienzusammenführung) passing the A1 exam is enough.
For most German learners, a course in a small group is the best choice. In a group there is lots of variation and you can practice many aspects of the language without getting bored. And of course group courses are cheaper.
If you want to have the teacher all for yourself because you have many questions or you prefer a very individual class, you can take one-on-one classes.
Face-to-face classes are very efficient, because a significant part of interaction between student and teacher is body language. In online classes nonverbal communication does not flow as effortlessly as in classes where student and teacher are in the same physical room together. It depends on your internet connection. If you have good internet and cannot attend a face-to-face class, an online class is a viable alternative.
Rolf Kampffmeyer, Munich - Germany, February 16th 2018