How do I replace this toilet shutoff valve?
That's most likely a copper pipe covered in solder. You can try scraping the solder off to see if you get down to some copper to verify. A copper pipe cutter is cheap and easy to use:



Get this onto the pipe a close to the valve as you can, tighten the screw until it's snug, and spin it around the pipe making sure to keep spinning on the same location (you're making a cut, not treading the pipe or making a spiral). After every few turns, tighten the screw a little more and eventually the pipe will break off. Clean the pipe with some emery paper, and install a new compression style valve (this only requires a few wrenches (crescent or fixed, not a pipe wrench or pliers if you want to avoid teeth marks). If you removed too much of the pipe, you may need to replace the escutcheon with a flatter style.