The answer is normal clothing. If you have any metal on your chest it will be removed prior to a CTA (such a jewelry) primarily so as not to cause any artifact. Plenty of water and hydration before the study as they will use IV contrast during the CT angiogram. Other precautions: One should not be allergic to IODINATED CONTRAST and the kidney function should be normal prior to any contrast exposure.
All the best.

Normal clothing, nothing specific. It is like having a CT scan and they likely will have you change to wear a hospital gown.