Teeth are not shell white and a lil yellowness is natural as the outer enamel thins off and dentin starts showing through the thinner enamel. Also food stains etc;
Next Steps
Teeth cleaning and polishing will restore part of your teeth whiteness. You can take appointment for same from a dentist.
And if you smoke, quitting will help reduce recurrence of stains. If you use mouthrinses daily, restrict it to using 2 weeks in a months and leaving it off for next 3 weeks or so.