Talking about the heart organ, I presume? And not the heart chakra.
Don't do asanas that can strain the heart. The Warriors, for example. As for strengthening the physical heart, this info is easily found online, and you can consult a yoga instructor who will find you resources. The program I made for yoga asanas is for the heart chakra, and can help with the heart organ but proceed with caution. Avoid the inverted asana, replace with waterfall aka legs up the wall. Link in my sig to my yoga posts, read the one on "Important Inversions" to see the one for waterfall.
To strengthen your heart, look into hawthorn herb, taken as a tea or otherwise. Also rose quartz. Program them specifically to heal and strengthen your heart.