How to get rid of discoloration around neck and on elbows&knees without effecting my skin. (21yr old female) also I have acne spots left after pimples?

Hyperpigmentation. You need to see a dermatologist who can make a recommendation for the treatment. The post acne pigmentation can be treated with peels, topical bleaching agents, topical antioxidants and more importantly, treatment for the acne. The other issue may respond to a moisturizer called LacHydrin (lactic acid) or AmLactin. Additionally it may be a sign of hormone issues, especially if you are overweight.