welcome to our community!
Here you can find a useful list of websites that you can use to search for apartments:
I also attached a "manual" on how to find a flat. I hope this information is helpful for you.
In Schleswig Holstein there is plenty of institutions and organisations that can help you with various questions. Some of them might be able to give you information about finding a flat.
Since you have no specific city in mind I can only give you an overview of the whole state. You can click on the map to get a list of information centers in the selected district:
Feel free to ask further questions if necessary :)