When I create a new post, two ACF fields are shown by default.
Is there a possibility to show these fields not until I select a category?
ACF allows you to create your own location rules http://www.advancedcustomfields.com/resources/custom-location-rules/
It should be possible to create a rule that checks to see if a category is selected of not. You could add a rule for “Category” “does not equal” “empty”.