The adverb ergo is a fancy version of "therefore." Use it as a connector between thoughts and sentences that logically follow. You were present during the robbery; ergo, you were called as a witness.
Ergo appeared in late Middle English meaning "therefore," probably from the Old Latin root regere, "to guide." You can see the relation to the word as it is used today, to present a connection between ideas where a second sentence or idea is "guided" by, or draws a logical conclusion from the first one. Satirist Jonathan Swift once wrote, “Words are but wind; and learning is nothing but words; ergo, learning is nothing but wind.”