I gather you are referring to the aerator where the water comes out? Most sink aerators have a thread allowing consumers to remove them when they get old. Depending on how old it is it may be hard to remove, but if you screw it anti-clockwise it should come out.
They are readily available from any Plumbing merchant or hardware store.
Aerators unscrew when turned in a clockwise direction when looked at from above. It is often necessary to grip the aerator with pliers in order to turn it. Placing a few wraps of electrical or masking tape around the aerator provides a better grip and reduces damage to the finish when the aerator is gripped with the pliers
A faucet aerator is usually attached to the end of a faucet.
Probably a clogged aerator. Remove the aerator and clean out the screen and reinstall.
You can put an aerator wherever there is threading for one.--I'm not sure of your description "brass faucet valve". - The end of the spout is generally threaded for an aerator.
Female x Female coupling
At the very tip of the spout.
A clogged aerator on the end of the faucet would cause the water flow to decrease in that faucet.
You need a special adaptor to link the two. This is an insert, threaded at one end to fit where the faucet aerator fits (you remove the aerator) and at the other end a pipe thread to fit a shower hose. These are commonly available in most plumbing stores and there are 2-3 variations. -Take your aerator along for comparison when buying the adaptor.
You have to have a faucet that has threads like you find on the outdoor faucet. Some have them and some don't. If you're trying to attach a hose meant for indoor faucets, you'll have to remove the aerator (the small nozzle on the end of the faucet) so you can screw the hose onto the faucet.
Normally at the end of the faucet spout
Unscrew the aerator and clean it and reinstall it
Clean the aerator
The water meter is shut off? The main water line is shut off? The valve for that faucet is turned off? The waterline is made of galvanized piping and has corroded shut? Aerator is clogged, remove and clean or replace.