Since you are a newbie, I'm not sure if you understand some of the basics. They have been discussed in this thread but not explained, exactly.
You have two water sources:
1. "City water" - you connect a hose to the city water connection and leave it turned on at the spigot. The pressure from the hose pushes the water to your sinks, toilets, etc - so turning it off will turn off your water. Most folks recommend buying a water pressure regulator so that excessive pressure won't burst the lines in the RV.
2. Your fresh water tank. You fill it with the hose, but it's not "connected", per se, and is not pressurized. In most cases, it doesn't even have a threaded connector - so you can't connect it and leave it running. Once you fill up, you are done with the hose and spigot. You have a water pump on board that needs to be turned on to provide pressure from the tank to the sinks, toilet, etc. Many will recommend that you turn the pump off when the water is not in use or at a minimum when you are sleeping/leaving the trailer. This is the tank that you should sanitize once per year as mentioned earlier.
Hope this helps!