Regular acne does not usually involve the scalp. A better name for "Pimples" on the scalp is folliculitis. Most commonly this is caused by humidity and sweeting. Thus, the first step for reducing folliculitis should be washing your hair every day and avoiding hats especially in summer. An anti-bacterial shampoo or a shampoo with tee tree oil can be beneficial. If you have a lot of dandruff you can alternate between a antibacterial shampoo and an anti-dandruff shampoo. More severe cases of scalp folliculitis should be treated with oral antibiotics such a minocycline.