OK, I have to weigh in here. Never hook up your water hose until after you have flushed the local water line! Unless, of course, you like drinking spider guts. Flush out the spiders and rust from the local line (especially early in the season). Then hook up.
And, in case no one has made it perfectly clear...hooking up to "city" water does not fill your onboard water tank. That is a totally separate operation. You have a fresh tank fill, usually behind a small locked door.
Happy camping. Actually, happy RVing. Camping is something different.