Muss ich bereits über Erfahrung im Schreiben verfügen, um eine Jorunalismusausbildung machen zu können?

Not at every school, but at many, you should already have professional experience in journalism in order to be admitted to a full journalism course. For the Bachelor's degree program in Communication, no further practical experience is expected if you have a vocational or technical Baccalaureate, but prospective students with a Baccalaureate must have at least one year of professional experience. This does not mean that you should have published extensive texts as a journalist, but you must have completed an internship of at least one year, preferably in the field of communication, although this is not mandatory.
In the case of short courses, however, participants are usually expected to have little or no experience, as long as it is journalism training and not continuing education, further training in journalism. Ask for detailed information on the respective courses or study programs so that you can also see the admission requirements.