I think that the color change in your teeth is most likely related to your smoking. Smoking involves exposing the teeth and the mouth to many different chemicals, some of which can react with the teeth and produce a variety of color changes, from yellow to dark brown.
I would recommend that you schedule a visit with your regular dentist
to discuss the issue. They will be able to examine your teeth to confirm whether or not this is in fact what is going on. If so, obviously, the main treatment will be trying to quit or cut back on smoking, if you can. Your teeth will probably also benefit from a professional cleaning, which your dentist's office will be able to do for you. In some cases, a whitening product might be recommended, although these often are not very effective if you continue to smoke.
Also your dentist can make sure that the discoloration is not due to a more serious problem, such as a cavity or erosion of the protective hard covering of the teeth. If so, this will require more intensive intervention both to get rid of the color and also to protect the teeth from further decay.
Please see your dentist as soon as you can.