Just being from a strong residency program, and thus also already having a good baseline application, you'll have your foot in the door and likely get an interview at most programs. Top programs are looking to train academic cardiologists. Start a cardiology research project now, or two, or three. Keep in mind this will be challenging to accomplish during your busy resident schedule. Make sure the project is DOABLE and that you have an excellent mentor. Carefully choosing the right mentor will make a huge difference in the ultimate success of the research. Do a cardiology elective early. Get to know the faculty. Design a cardiology-oriented selective. Start thinking about what kind of cardiologist you want to be (imaging, electrophysiology, interventional, heart failure/cardiomyopathy, CICU, etc). On your interview day expect to be able to present a 5-10 year career plan and a general plan for your research during fellowship. If you can legitimately pull of the latter, this will make you look like a rock star. Again, the top fellowships want to train academic cardiologists and they want to choose fellows that have a high pre-test probability of succeeding. Convince them on your potential for success.