Having trouble with excercise!?

I weigh around 300 pounds and i've actually lost weight with my new diabetes medication and totally changing my eating habits. I became a vegetarian, cut out soda, and all sweets that I used to eat. If I could just start an excercise routine, I could loose even more. I smoke so I'm kind of afraid of a heart attack with something too strenuous. Any suggestions are helpful, thankyou!