There usually is not a shutoff valve for just the shower. The toilet has it's own shutoff, and sinks have their own shutoffs under the sink. But tub and showers typically do not have their own shutoffs. There's always the exception, like in my own house, where everything can be isolated. If your house has a basement, go in the basement and trace the plumbing lines. An easy place to start is with the hot water heater. You'll usually see a copper line come off the hot water heater and run off towards the kitchen and baths. There will typically be another copper supply line running next to it, that's the cold water supply line. As these supply lines run across your basement, there will likely be a few "tees" where secondary lines branch off from those primary lines. One set might go to your kitchen, one to the laundry, as well as a set to each bathroom. Where they branch off, hopefully there is a shutoff valve on each line that allows you to shut off the hot and cold water to each secondary set of lines. Your best bet would be to do the demo and expose your plumbing that needs to be replaced. Then shut off the water and replumb. It should only take a few hours to replace a shower valve if that is your intention. You'll then take a 4" long galvanized nipple with threads on both ends. Thread a cap one one end and screw the other end into whatever plumbing outlets you have in your shower. For a shower head it'll screw into the same brass union that your shower arm screws in to. Now you can turn the bathroom water back on, and even if the shower valve was accidentally turned on, no water would flow out of the capped nipple. If your house is on a slab, the shutoffs can be located in different areas. I've seen it vary by region. If you plan on keeping your existing shower valve, then just unscrew the shower head and arm out of the wall. Screw the capped nipple that I wrote about earlier in to the place that the arm came out of. Then remove the shower valve's cover plate and trim pieces and you should be able to demo to your heart's content. One note...whenever you use any threaded plumbing materials, like threaded nipples or shower arms, etc, use either teflon pipe dope or a couple of wrappings of teflon tape on the male threads before you screw the male threads into the female fitting.

If it continues to drip after the water has been turned off for an hour or two, I would think that the shutoff is not 100%, but that is common as these valves sit and don't get used for years on end so they can be less than perfect years down the line. It doesn't mean it had to be replace, necessarily. However, that's not the main issue. If you have drips coming down the riser (and they are not condensation), then you have a small leak above, probably where the riser is connected to the faucet. Get under there on your back with a flashlight and try to determine the actual source of the drip. Unless the faucet has corroded, I'm betting that simply replacing the riser with a flex hose will fix this.

How about a follow up -- a way late follow up ... So the filter is supposed to be a whole home filter, because the city has super old pipes with a lot of sediment. However, the filter needs to be changes monthly, and my sister had no idea. She (unknowingly) let it go for years. It was so backed up that her house barely had water pressure. It should have been an easy fix when I went to help her, but the 100+ year old house had a faulty main shut off valve -- as well as the pre-filter valve being faulty. So she had to call a plumber to contact the city to shut off the water at the street/sidewalk. There was nothing I could have done from there, the city won't turn it off for a homeowner (unless they don't pay the bill, lol). She now gets 'subscribe and save' monthly filter deliveries from Amazon to remind her to change the filter. Luckily, the plumber was able to blow the back up sediment out of her pipes, and she didn't need any pipe replacement. Her water pressure is now back to the usual crummy-100-year-old-house water pressure, instead of the even worse pee dribble pressure that she had with the backed up filter and pipes. Thanks!

Well I learn something new every day! Thanks for that explanation, Jake. I googled it and see what you mean about the tapered vs not. irafikov, it you want to do it for experience sake then I'd suggest trying the compression fitting. It isn't that hard to do. The point of the push-on types is that you are spending more for something that you don't have to learn with or think about. It's almost idiot-proof. Cheaper than hiring a plumber, though. And since you already have it, just give it a go. (Actually, maybe that's my pre-COVID advice. If you can't figure out the compression fitting, you could always pop out to the store to buy the other push-on type. In this day and age, if you can't figure out the compression fitting, you'll have to leave the water turned off until you can pre-order and go and pick up the other fitting which can take ages) If you really want to learn, then I'd suggest trying to solder a new valve on. It'll be more expensive if this is the only plumbing you ever do (because you have to buy all of the initial equipment and materials), but in the long run it'll be cheaper per fitting and the best long term fix IMHO. But this is only if you are typically a 'handy' kind of guy. If you don't do other types of DIY around the house, then I wouldn't suggest sweating copper as the first task that you try to do. When attempting to solder a joint, you could burn down your house or cause a leak. With a compression fitting, you can only cause a leak. :)