I am unfit and overweight. i am a 28 year old female. i weigh 14.5 stone. 5ft 6in. i want to get better. but even casual walking can make my heart go up to 125 bpm. i'm scared if i exercise i'll drop dead. how can i get fit and healthy safely?

Is there any way you can qualify for physical therapy? They could start you off with exercises you are able to do and then gradually build up as you tolerate. And, they could give you exercises to do at home that you definitely tolerate. There is also cardiac and pulmonary rehab. Your doctor would be able to set you up with one of these, most likely, as a way to start an exercise program safely.