Dark neck armpits inner thighs & cheeks.

I had very clear skin until the age of 10.I used to be in the sun alot playing ball etc.I have dark spots on my cheeks,My armpits are Dark,my knees ,elbows are dark and My neck.is dark with lines thru it also my inner thighs including between my breasts even,my hips are dark and spotty , even my panty line from my waist is dark .I have discolored skin and I dont know why my skin turned out so discolored is there anything I can possibly use what wont damage my skin,But give me my clear brown skin back please.Its very embarrassing,if I wear tanktops my chest is bright and my neck, armpits ,and face dark. also I have spots on my face.by the way I do have dark patches between my arm creases can you please tell me whats going on with ny skin and what can I use ?? Thanks Alot.