Your heart is a complex and robust organ that is obviously very important to living a good quality of life. Given its complexities, there are many problems that can occur -- so it is difficult to say exactly what your weak heart is specifically, and therefore difficult to discuss how to improve in. I would recommend that you discuss this with a heart specialist
, known as a cardiologist.
In general, a weak heart refers to a weak heart muscle. This is most commonly caused by heart attacks, but there are a number of other causes. In general, the best way to strengthen the heart starts with preventing further weakening. For example, if the weak heart muscle is caused by heart attacks -- the best way is to take medicine to prevent further heart attacks.
There are also many medicines (depending on the cause of the weak heart) that can improve the heart muscle. For example, ACE inhibitors are a class of medicine that can help the heart.
There are also some very well proven exercise to improve the heart. These exercises can be learned in a process known as cardiac rehabilitation (or cardiac rehab). I would strongly recommend this as they will show you your own personal limit. Again, please discuss with your cardiologist.