They say you get as much as you put into every clinical opportunity you get.
As a first year, it's great that you'll be shadowing a surgeon, but it may be difficult to engage much with the experience as you may not necessarily have been taught a lot of the things related to cardio-thoracics/ surgery.
For a start, you could go over the anatomy and physiology of the heart. Learn the vessels that supply the different chambers of the heart and go over some of the physiology of the cardiac cycle. This should give you a good grounding to understand what you'll be observing in theatre, and also might give you a few intelligent questions to ask the surgeon (or answers to his questions if you get grilled!)
From a clinical perspective, go over cardio examinations and history taking. Surgeons love medical students who see their patients BEFORE the surgeries, so try and make your way up to the ward and ask the junior docs if they have the theatre list for the day / the following day and ask them if you can speak to them when they come in for their surgeries.
In addition, you could ask the surgeon you are shadowing if it's possible to sit in during a pre-op assessment clinic. This is probably going to be the most useful thing for you as a junior medical student as you'll observe important things that will be more relevant to your future OSCEs e.g. Medical assessment of a patient pre-operatively, obtaining consent for a procedure. Also, this is where the surgeon will usually explain the procedure to the patient in layman terms, so it's a great way of learning a little bit about the procedure, as well as how to communicate with your future patients so that they will understand you.
Hope this advice helps! Don't feel like you have to be on top of all the knowledge of things though. You are still in 1st year and you really should enjoy it as much as you can!
TL;DR: Skim over cardio anatomy/physiology before attachment. Try and speak/examine patients before their operations. Attend pre-op assessment clinics if possible. Don't stress, enjoy med school!