In order to be a cardiologist in the US, you would first need to complete a 3 year Internal Medicine (IM) residency. During your second year of that residency, you would then apply for a 3 year Cardiology fellowship.
In order to be competitive for a US residency as an IMG, you will need to:
1. Complete USMLE Step 1, 2CK, and 2CS. Doing well on these exams is a very high priority.
2. Obtain some US clinical experience. The best way to do this is to arrange for an away rotation in the US while still a medical student in your home country. Once you graduate, it is very difficult to get any clinical experience in the US.
3. Apply broadly to programs in the US. This is quite expensive, once you consider travel costs etc.
Cardiology is one of the hot fields in the US now. IMG's have difficulty getting spots. You will need excellent USMLE's, and much will depend on the residency you obtain. There is some chance that you could get an IM spot, and never get a Cardiology spot.
Good luck!