Scratch

Hi! I'm totally new to Scratch and have several questions about a game I'm trying to make. It starts with a title, then the player clicks to switch to a background w/ brief instructions, then it changes to a background where the player chooses a letter from the alphabet.
Each letter is a sprite, and when clicked, it changes into a monster (starting with that letter) and all other sprites disappear.

Here's my first issue: I want all the sprites to hide until we get to the third background...

Previously someone posted this:
"My game needs to hide a background and make a sprite only show on a certain background."

For which scimonster recommended:

forever
  if < ([background #] of [Stage]) = (wanted background #) >
    show
  else
    hide

... but I'm not sure what this means! Do I assign numbers to my backgrounds? If so, how? If not, where do I get the numbers come to insert into this code?

More questions to come later if that's ok!