Anything, everything and something play so many roles in the English language, I sometimes think of them as members of a small acting troupe. They can display endless versatility, depending on the context in which they’re used.
Anything is a pronoun that can refer to any object, event or matter. You’ll use it when you’re unable to be specific or have no intention of being so.
Example: Barry will steal anything that isn’t nailed down.
You’re saying that Barry doesn’t care what he steals; if he sees something he wants—regardless of what it is—he’ll take it.
Example: He asked me what I wanted for Christmas, but I couldn’t think of anything.
Here you’re saying that nothing came to mind; you’re unable to be specific.
Everything is a pronoun that means to encompass all.
Example: Jim was so hungry, he ate everything on his plate.
In other words, he ate it all.
Everything can also mean an essential fact.
Example: Mary’s friendship meant everything to Frank.
In other words, her friendship is of utmost importance to him.
The word something, when used as a pronoun, refers to an undetermined or unspecified thing.