I have uneven skin tone. my skin tone is medium. my face, belly, legs, armpits and knees are darker than the rest of body.how can i have an even skin?

It can be hormonal: Some medical conditions can cause hyperpigmentation in certain body parts. People with Insulin resistance who are predsiposed to develop diabetes, or even diabetic people can develop darker pigmentation. Make sure that you are screened for diabetes and have your Insulin levels checked. Another possibility is genetic predisoposition. Certain races, especially ethnic ones, have those features.

Go see a dermatologi: I would recommend seeing a dermatologist to diagnose your condition.