Reading a multimeter: the five settings you actually use and what the numbers mean

DC volts (batteries, car, anything with a plug-pack), AC volts (outlets, only if you are confident), continuity beep (is this wire connected to that wire), resistance (is this heating element / fuse / coil open), and diode mode (is this LED or diode dead). Probes: black in COM always, red in V/Ω for all of those. Never measure resistance or continuity on a live circuit. Never put the red probe in the 10A jack and then measure voltage; that is how meters and fingers get hurt. Auto-ranging meters remove the one mistake beginners make most.