start here: https://www.google.com/#q=stuck+bicycle+stem

By the way, WD 40 is good for removing glue remains from stickers, light lubricating situations like door hinges, etc. It is not a penetrating or rust desolving chemical and is almost always of little help in removing rusted stems, nuts, bolts, etc.

Everyone has their personal favorite, mine is PB Blaster.

For stuck situations, I clean the area as much as possible so that the blaster can get right to the rust. Wait a few minutes, tap the stuck item with a plastic tipped hammer, and then try to remove stuck item. Reappy chemical and repeat process from time to time. Can take 24 hours or more if you want to avoid damage.

Links above are more informative, but ditch the WD as a first step and get a penetrating oil.