When you place a semicolon in a sentence, remember that you must have an independent clause both before and after the semicolon and that the ideas expressed in both main clauses should be closely related. Example:
I like you; you’re nice.
Also remember that the semicolon is always used before a conjunctive adverb that introduces a second independent clause. Example:
Her arguments sounded convincing; therefore, the majority voted for her.
The word therefore is a conjunctive adverb. Note that a comma always follows the conjunctive adverb.
Conjunctive adverbs include accordingly, also, anyhow, as a result, besides, consequently, furthermore, henceforth, however, indeed, instead, likewise, meanwhile, moreover, nevertheless, otherwise, then, thus, and therefore.
Most editors agree that the use of semicolons should be kept to a minimum in fiction. I wouldn’t use them at all in dialogue. The University of Chicago Manual of Style, the bible of the publishing industry, says, “Semicolons tend to be frowned upon in fiction. An editor who doesn’t allow them at all is overly rigid, however, since they are sometimes useful and even necessary.”