I am replacing my toilet tank. Sounded easy enough until I tried unsuccessfully to completely close the shut off valve under the toilet. Rather than attempt to replace the valve,(I am an inexperienced d.i.y. where plumbing is concerned), I am thinking about shutting off the main water line,& proceeding as planned. The main is located in the basement, and the toilet in question is on the 1st floor. Do I turn the main off first, and then open a faucet to relieve pressure or in reverse order? Does it matter which faucet or faucets I open? My house is over 50 years old. What are the chances of my damaging the main shut off valve and last would it better to get a little wet,& NOT go near the main shut
Off? I really am not paranoid but just overly cautious, and a little nervous.

OK, Does you main shut off valve work?? Sometimes those old valves get stuck. I'm guessing it's a gate valve... What kind of valve is it? If you can't turn it by hand try to turn it gently with a pipe wrench to loosen it up.

If you get it closed off then you are set. Disconect you olt toilet tank hose and let it drain to a bucket.

I would recommend getting that valve fixed at some point. I've never had luck replacing gaskets in angle stops.

Thanks for the quick reply.