I've recently gotten Photoshop and was wondering how everyone does the white highlights around the horses body, I just cannot figure it out.

Around the body, you want to right-click on your base layer, and choose 'blending options'. Then go to 'outer glow' and tick it on, then use the setting options to adjust it. Select the colour you want, by using the eyedropper tool and choose a colour coming from a light source on the image - if you don't want to do that choose a white that leans vaguely towards the main colour scheme of the image. This should do it for you :)