ere are two basic rules for the use of semic-colons:

A semi-colon is placed between two independent clauses. For example,

Procrastination doesn't have to happen; a writer must decide to delay.

Note that two, shorter sentences can be joined by a comma. For example,

He started early, success soon followed.

A semi-colon is placed before a conjunctive adverb (however, thus, therefore, consequently, etc.) when it is used to join two sentences (independent clauses). For example,

I wanted to start writing sooner; however, I never quite got around to it.

Note that a comma is placed after a conjunctive adverb.

For a more detailed and more formal explanation, see The New Century Handbook or a similar source.