Irony

Irony: the use of words to convey a meaning that is the opposite of its literal meaning.

Irony can be used to inform or amuse the reader. They're not always obvious to the writer (most of its unintentional.) There are three types of irony; situational, dramatic, and verbal.

Situational Irony
This occurs when there's a contrast between what we think should happen, and what really happens.
This is the big twists you use to surprise and entertain the reader. I highly encourage twists. Predictable plots should stay with the fairy-tale genre. (The girl gets the prince? Wow, didn't see that one coming.)
Suppose you, the writer, were to create a big buildup around a certain character, casting them in suspicion and secrecy. Then at the end it turns out they're innocent, and even noble. Situational Irony.

Dramatic Irony
This is one of the more popular ones. This is when the audience knows something the characters don't. Think Romeo and Juliet. ("She's alive you fool!")
This is next to impossible to pull off in first-person point of view. Even in third person limited it could be difficult. But it's always tense and climatic when achieved successfully.

Verbal Irony
When the writer or a speaking character says one thing but means something else. In a word; sarcasm.
Sarcasm is encountered almost daily. (For me anyways. You might live in a nicer neighborhood.) I already provided you with a nice example in the situational irony blurb:
'The girl gets the prince? Wow, didn't see that one coming.'