What must be my first step toward becoming more active after retirement?

Deciding to take it! Your decision to become more active was the first step to becoming more active will lead to another step and another! good for you for identifying that as important. If you haven't been that active, start slow and ease into it. You might benefit from some direction from a trainer, and you might check in with your doctor to see if s/he has any concerns or advice. Best wishes!