Get a professional cleaning and mouth exam. You might only need a thorough cleaning to restore your smile's sparkle.
Your dentist will also look for cavities and check the health of your gums during the exam. Treating any problems before you whiten is safer for your mouth.
Ask your dentist about which over-the-counter system to use and how much lightening you can expect. Teeth do darken with age, and the amount of color change varies from person to person.