The above's got it- Let's Plays, Walkthroughs, and good ol' Trial and Error!
A lot of it is simply learning the gimmicks and tropes of a genre from scratch- only time & experimentation can give that. Using Final Fantasy 7 as an example: the basics are simple. You can see you character's health number on the screen. Choose attack/defense/spells to keep yourself alive & kill the monsters. The hard part is learning about the actual commands. How much damage does your attack do? How do you heal? How MUCH do you heal? What does it take to use a spell? What does it do? How much damage are the enemies doing? These are all things you need to pick up on to succeed. Prior knowledge of RPGs helps- Potions & herbs heal, Use thunder magic on metal enemies for more damage, and so on.
A shorter way to say it: The manual can tell you 'right trigger fires your gun'. Experience tells you 'shoot the red barrels and they explode'. The only question is, are you enticed by this unfamiliar game enough to put in the time to learn how it works?
This reminds me, I need to pick up Driver: San Francisco again.