if the unsupervised classification took too long you can
- subset your image and test it (a whole S2 scene could take really long for a cluster or EM analysis)
- reduce the number of clusters
- Choose another classifier (EM and K-means take different time)
For a supervised classification you have to define geometries including your training areas. 
Make one per class you want to detect.
They are stored in the vector Data folder (don’t mind about pins and gcps)
You can then select a supervised classifier, but make sure you know a litte about how they and corresponding parameters work. KNN is quite simple and you can’t do much wrong with it. Your trained classes should appear in the dialogue:
