My son and I would like to play a Steam game in multiplayer. How do I set this up? Do I need two Steam accounts, or is this possible with one (Family) account?
You do need two Steam accounts: a shared Steam library (through Family Library Sharing) can't be used simultaneously by two (or more) people. As per Steam's FAQ:
Can two users share a library and both play at the same time?
No, a shared library may only be accessed by one user at a time.
Setting up a local multiplayer game is dependent on the game, and usually this can be done relatively easily in-game.