There won't be any problem at all.

First of all you must discontinue smoking. You are just 21years old. Stopping the habit followed by scaling n polishing will help you a lot.  If the stains are very stubborn you might need whitening of teeth. But all this will help only when you stop the habit. There is no point in making your teeth neat n clean n sparkling & then making them black n dirty again with smoking.

there won't be any problem.. may be a lil sensitivity.. for a while, that can be taken care by any anti-sensitivity toothpaste.

Hi No major problems except pain for day or 2....but you should stop smoking otherwise it will reccure...consult good dentist they will do the needful

plz quit smoking & go fr scaling ,polishing ur tooth as soon as possible . it has no sideeffect at all as my advice to u . go fr scaling in regular interval.

scaling or oral prophylaxis is the best solution for the black spots. usually no problem after scaling if done properly...in some case you might have a little bit of hypersensitivity for a few days, that resolves well with the common anti sensitivity toothpastes... you should also consider stopping the smoking habit. because stains are just a minor side effect of smoking. there are bigger problems like cancer attached to it...your dentist can also help you if you want to quit smoking