Smoking is a massive risk factor in the development of cardiovascular disease and it dramatically increases your chances of developing lung cancer.
Normal, well functioning lungs are essential for safe diving to minimise the chance of pulmonary barotrauma (burst lungs).
The simple advice is to STOP SMOKING. Your GP or local NHS stop smoking service can provide you with support.
If you have suffered ill health and smoking has been implicated as a factor (e.g. a heart attack), there is very little chance you will be found fit to dive unless you have stopped smoking.