Pimples on the forehead and other areas of your body are caused by blocked pores in your skin.
Over production of oil from the sebaceous glands leads to the blockage of hair follicles and when those hair follicles and pores get blocked pimples start appearing.
To get rid of pimples on your forehead you should wash your face twice daily using a gentle cleanser and doing so will get rid of the excess oil from your skin.
You can also help get rid of the pimples by applying some Lemon Juice or even apple cider vinegar to the pimples and letting it sit until it dries.
Keeping your skin exfoliated and using some cream for acne that contains ingredients like benzoyl peroxide or salicylic acid can help get rid of the acne and keep the acne on your forehead away.