I have big forehead; what should I do to decrease my forehead size?

WIthout pics it's difficult to tell exactly what you need but there is a forehead shortening procedure. 

In this procedure the air is advanced down the forehead usually 1-2cm to reduce the size of your forehead. It all goes back to balance. The face should be porportioned equally in 3 areas. The first area is the hairline to the area between the eyebrows, the next is from the eyebrows to just below the nose, the third is just below the nose to the bottom of the chin. those should all be equal in size for the eye to see beauty in your face. 
this isn't exact but it's pretty dang close!
Hope this helps and God Bless!
Dr. Robb

You did not state if you are male or female. If male, a forhead reduction is not the way to go. There are two types of procedures for lowering a woman’s hairline. One, is a surgical excision of the upper forehead followed by the downward advancement of the hairline up to an inch from where it was prior  =to the surgery. To accomplish this, the entire scalp from the frontal hairline to the very back of the crown, is lifted up and pulled forward. When this is done by an experienced surgeon, the hairline can often be advanced about 2/3rd – 1 inch if there is enough looseness when this lifting process is done. This procedure is often complete in one surgical session; however, it may not address receded corners of the hairline if the corner recession is significant. A great advantage of this surgical approach is the immediate impact of the procedure on hairline location with healing usually complete in a week or so. Sometimes this procedure produces a scar in front of the hairline, and if this is this is the case, a hair transplant in front of the hairline can address the scar. If there is corner rounding that is not addressed by the surgical lowering, then hair transplants can address this very well. Hair transplants take 7-8 months to fully grow out.