The first time you start KeePassXC, you will be prompted to create a new database to store your passwords or open an existing one. (You can also create new databases later using the menu item "Databases").

You can keep the default settings for database encryption:

Now you have to choose a master password that protects all other passwords. You can also create a secure random master password by clicking on the cube  in the following dialog. Remember: You must enter this password if you want to access the other passwords in the database. If you forget this master password, you will not be able to access the other passwords. It is not possible to set a new password without knowing the old one. (Your institution may have regulations for keeping copies of such documents, e.g. in a sealed envelope in the safe of the business room).

Select the location for your passwords encrypted with the General Password. Depending on whether you need the passwords on several computers, it may be useful to select a folder synchronized to the cloud of Kiel University.