Watch, ask questions, read and study, ask questions of everyone (techs, nurses, Cardiologists), learn policies and procedures. Each day you'll learn more and more, and finally one day you'll realize
"I still don't know it all!!!! WAAAAAAAAAAAAAAAAAAAHHHH!!!" (but you'll be a lot more skilled and know a lot more than when you first started).
(your question is a little hard to answer without specifics: your background experience [any ICU/CCU experience?], size of lab, the scope of what you'll be required to do, who is training you, how long is orientation, what kind of a lab is it [diagnostic-only VS interventional, and will you be helping with ONLY coronary procedues or other vascular procedures as well, . . . etc)