What do I have to pay attention to during the designing of a system?

Unfortunately there is no "correct system design".
Complete knowledge of the installation location is decisive for a good system design. This includes amongst other things the location, orientation, inclination, possible shadowing of the solar module as well as the exact module type etc.

This information should be compiled and evaluated by an expert (solar technician / installer) on site. The expert can then recommend a sensible system design.