Professionally. If you can't remove tea (or any other) stains from your teeth with normal brushing and flossing and the use of some ada accepted toothpastes, then you need to see a dentist for a professional cleaning. You should be aware that many "whitening" toothpastes are too abrasive to be used routinely on your teeth and can actually do damage.
Whitening toothpaste. Whitening toothpaste can help remove tea stains. A professional cleaning at a dental office will remove most tea stains as well.
White strips. Teeth whitening strips are very good at bleaching stains.