I want to shut off the water valve so I can replace the mechanism inside my tank. I've never touch the valve and it won't move, it seems to be frozen. I don't know what I need to do.

You could change the shut off valve, but if that is something you don't want to tackle, you could also simply shut off the water main to the house, drain the pipes a bit and then install the new tank mechanism. After installation of the new part, turn the water back on, purge the air out of the water lines and and test your work.

How's that sound?

Mark

It sounds great but not for me, I think I may need to get a Plummer over to do the valve and add the new fluid master to the tank. I even tried using a little WD-40 to see if that would loosen it but no.

"Overshoot", a nice term for missing the bowl, corrodes the die cast and it seizes the shaft. Loosen the packing nut just a bit, apply WD40 and use a pair of Channel locks turning the handle to the left. If you really have to strain, let a plumber do the job and replace with a 1/4 turn stop valve..