Hi, as your question is for whitening of tooth, Leme be very practical, things would actually become bad if u try to whiten your tooth. Tooth sensitivity would start then. Yours is a developmental defect as u r aware of ( since it was there from your childhood). This condition is fluorosis with surface disfigurement. But there is nothing to worry as there are multiple options to correct this. Your enamel is mottled. You can visit a cosmetic dentist in your locality and can plan out for complete esthetic makeover.
Next Steps
Best is to visit a dentist to plan for a cosmetic correction. The earlier will be better as waiting is no longer Gona make things better in any way.
Health Tips
The biggest word of caution is don't try anything by yourselves to remove your brown stains just by seeing internet.DIY( do it yourslelf ) is very risky. Don't try to scratch your tooth with any sharp objects as it would make the situation worser. Don't worry your current situation can completely be corrected. Visit a specialist soon