Semicolons are used to connect two independent clauses that, though they could stand on their own, are closely related.
Example: Call me tomorrow; I’ll have an answer for you by then.
If there’s a coordinating conjunction beween the two clauses, e.g. ‘but’, or ‘or’, then use a comma instead.
Example: Call me tomorrow, and I’ll have an answer for you by then.
Tip of the week: Semicolons.