Thoracotomy: i'm wondering if this is a complicated surgery?

Depends: It depends on the reason for the surgery, as well as the operator. There are different approaches for thoracotomies. The older approach requires extending up the back, while the other is called a muscle sparing approach. A lot depends on training.

To do what?: What determines complexity is the actual operation done after the thoracotomy. Thoracotomy is the name for opening the thorax. The important thing is what is to be done inside the chest nonce the chest has been opened.