How to stop smoking habit. Leave me?

Its bad. I get that. Initially my friend introduced me and taught me how to smoke. Its been 5 years we are smoking regularly. We have turned to be chain smokers and we both are worried. Trying to give up this bad habit but smoking is not leaving us. We are consuming 15-20 cigarettes daily and it has became so habitual that we always want more of it. Problems like cough and cold is not leaving our body. We feel Mild chest pain sometimes too. I dont know if it is too late to leave this bad habit. Any minute disease and i gets worried if it a cancer. I have already tried the nicotex tab to get rid of this habit but no result. Can someone please help? How to leave smoking or let smoking leave me?