Which level should I start with?
All learning materials are customized according to the Common European Framework of Reference for Languages (CEFR). It calls "der Gemeinsame europäische Referenzrahmen für Sprachen (GERS)" in German. You may find out more about CEFR and GERS with the links at the bottom of this article.
The language level indicates the language skills of a student.
B2 Upper intermediate
If you have never learned English before, you should start with the A1 Beginner level.
Most of the German learners have learned English in schools, their English knowledge would be at least at A2 level. However, many learners did not use English for many years and they would like to practice their English again for personal reasons. They should take the A2 Elementary course.
If you are comfortable using simple sentences expressing yourself and would like to brush up your English, so that you could share your experiences and make new friends in your travel, you are ready for the B1 level.
For the learners who use English at work, or would like to work in an international company, you should have at least B1 level of English. In B2 class, you would learn how to use English in interviews, presentations, and to communicate with native speakers and so on.
Here is the link to test your English level.