Due to the shortage of IT teachers in Italian schools, the teaching of computational thinking is carried out by in-service K-12 teachers from scientific areas not specialised and by novices in computer science (CS). It is crucial to investigate not only the training of teachers in digital skills, but also how their beliefs, attitudes and behaviours can affect, in detail, their implementation in the classroom. From these premises, the present exploratory study investigates the self-efficacy beliefs, intrinsic motivation and perceived effort of a group of 46 teachers who, on a voluntary basis, engaged in a 20-h workshop on CS teaching. The results show a significant improvement in self-efficacy, despite their perception of strong effort to master the subject.