Your only hope is a good penetrating oil. Do not use WD40 since it is supposed to be water soluable. Even if you get it free, it is time to replace the valve. This problem is going to resurface, probably when the toilet is overflowing and you really need to shut it off. It's a very cheap and easy fix......

In most cases you should be able to remove the toilet shut-off valve AFTER you shut off the water at the whole house shut-off valve.

The valve should come off fairly easily with the use of a couple of wrenches or a wrench and vise-grip or pipe wrench. Use teflon tape to wrap the threaded pipe coming through the wall and tighten the new valve snugly. Don't buy the cheapest valve available because there are definitly poor quality ones out there.

If you have trouble getting the old valve off let us know. I'm sure someone will offer advice on how to really butcher up your plumbing so you can go without water for a week or two.