I live in a roughly-50 year-old apartment building and would like to renovate the bathroom myself. I've tiled and replaced sinks and toilets in the past, but I'm unsure about replacing the tub and shower fixtures (knobs, spouts, etc.). Will I be able to shut off water to the tub without needing to shut off water in my whole line of apartments? I can find a [third] valve underneath the neighboring sink that appears to control the hot water flow to the tub, but no clues about the cold water.
Given that it's an apartment the plumbing would most likely have to be done by a licensed plumber, check with the landlord/building manager and local government.
In some places the work can be done by anybody, but must be inspected at different stages by a licensed professional and/or the local government.
There would be a small door with a valve somewhere, either inside your apartment or elsewhere. If each apartment is metered separately, there will likely be a valve by the water meter.
Even if the water needs to be shut off for the whole building, it will only be needed for a few minutes while you (or a plumber) removes the faucet and plugs the pipe, then again to attach the new faucet once you're done.
Some states (e.g., Massachusetts) require that most plumbing work be done by a plumber. Call your local building department.