@StefanPlanning a good basic example of a Decision Tree with some description you will find in the Beginners Space of the KNIME Hub:
To score new data you will have to provide the same data with all the columns as before but without the (unknown) target. So for example a collection of customers where you know their ‘features’ but not if they want to buy something.
If you want to learn more about machine learning and KNIME I have collected these hints and links: