Can i apply a SPF-15 Fairness lotion, even wen i don't step out at all in d sun ? Will SPF harm in that case ?

No, It will not harm you, SPF 15 is pretty normal, SPF 30 is the harsh one which would be required by cities with tropical weather conditions, especially summer or if your lifestyle requires you to be extensively outside exposed to the sun, as for you applying it at home, there would not be any harm in it, many people might not know, but even sitting to long in front of the computer to could make you dark due to the radiation from the screen, at that point of time a Sunscreen lotion is always advised by any dermatologists incase your exposure to computers is on the stretch more than 2 hours per day.