it’s all down to personal preference and style (John Williams classical- thumb on the back of the neck). I’ve been playing guitar for 26 years and for the most part my thumb rests on the top of the neck. If I want to play a particular stretchy chord, I’ll shift my thumb to the back of the neck. Practice is the key. Discuss what sort of music your into with your tutor and once you have the basics down you can start learning a tune you like. Keep it interesting.