Sassy, it appears that you have an unhealthy scalp, it might be seborrheic dermatitis. If the symptoms are scalp irritation, scales on the scalp or red patches you need to see a dermatologist for evaluation and treatment. I advise you not to put any peroxide or dye in your hair until your scalp has healed.
If your hair is brown and you want to be blonde, the 20 volume developer isn't strong enough to lighten your hair to blonde. You may need to use a 30 volume peroxide to strip the brown color from the hair, then a toner has to be applied. It's important to wait until your scalp is healthy before dyeing your hair. When your scalp is healed, please do not attempt to dye your hair on your own, it's best to let your hairstylist do it for you.