Hello sir.... To make your teeth strong and whitened you need to go for scaling ( cleaning and polishing) regularly once in 6 months... the scaling will removes extrinsic stains and deposits like plaque and tartar,stains of tea,coffee, beverages... sensitivity may be due to many cause **Dental caries **cervical abrasion of tooth due to faulty tooth brushing technique **Occlusal erosion due to bruxism (teeth grinding) **Gum recession which exposes root due to faulty tooth brushing or due to trauma from occlusion..... treatment is based on cause of sensitivity... In cases of dental caries ,occlusal erosion and cervical abrasion filling ios needed... in case of bruxism then occlusal splint is needed

If u hve sensitivity problem first start using toothpaste for sensitivity... Many brands r available Secondly sensitivity is not so common in young age like urs....so u need to visit a dentist as soon as possible to diagnose the problem and cause. And for whitening of teeth  whether u need cleaning or bleaching it depends on ur oral health... For that either send pics or again visit a dentist