Make sure to find out what resources your program has readily available. No sense in buying things they will provide.
One of the hardest things for me was visualizing the anatomy and understanding the blood flow pre and post surgical repair. The Field Guide to Congenital Heart Disease and Repair (Everett and Lim) has great pictures. I think there are a couple variations of this type of book and it's worth flipping through.
Nichols is the cardiac physiology textbook I've seen used most often. The first couple chapters really get into cardiac output and shock physiology which is vital. The ECMO red book also has a fantastic (and short) chapter on oxygen delivery. Really in the end, you need to master O2 delivery and shock along with cardiopulmonary interactions. If you can get a grasp of the DO2 equation (DO2= CO x Hgb x SaO2 x 1.3 + 0.003 x PaO2) and the factors that effect it, you'll be in a good position to start.
But there is time, and you will be taught. Don't panic about feeling unprepared.