Hopefully the club you choose will have instructors. Getting your instructor's advice on plane/engine/radio is a really good idea. Also, if someone in the club has a trainer plane for sale, the instructor can inspect it to be sure it is worth buying.
Popular first planes in the US include
Sig Kadet LT-40
Great Planes PT-40 or 60
Tower Hobbies Trainer, 40 or 60 size
Hobbico Superstar, Hobbistar, or Avistar
Thunder Tiger Trainer, 40 or 60 size
Hangar 9 Alpha or Arrow
There are literally dozens of excellent trainer airplanes. I have no idea what's available in Germany but I'd imagine there are a bunch of good ones we've never heard of.