Tooth decay is usually caused by prolong retention of any sticky food containing sugar, frequent intake of sugary food/drinks & not rinsing your mouth after eating. Do brush twice daily & avoid snacks inbet meals & rinse after every meal to avoid decay.. If you having teeth sensitivity, consult your dentist for your gingival health & even to rule out whether it is localized to one tooth or generalized accordingly treatment will differ . If localized to particular tooth, its because of the exposure of dentin layer of the tooth, the the reason has to be found out & immediately get it treated by your dentist. If it is generalized you can use any sensitive tooth paste with soft tooth brush & do massage your gums well & do get your teeth cleaned every 6 months for healthier gums.