It’s not that difficult a concept: If you have a sentence that could be divided into two sentences by removing a conjunction (a compound sentence), there must be a comma before that conjunction. This isn’t optional. This isn’t a matter of personal taste.
It is also a good idea not to use a comma after a conjunction-type word at the beginning of a sentence. If you’re going to start a sentence with but (usually fine in informal writing, which is what fiction is), don’t use a comma after it. The same thing goes for and. After all, if you use but or and in the middle of the sentence, the comma goes before it.
Inept punctuation in a novel makes the author look bad; inept punctuation in an indie-published novel makes every other indie author look bad, too, because a lot of readers still think indie equals unprofessional. Perpetuating this misconception is not acceptable. Learn to write with at least a minimum level of competence in the basics of punctuation, grammar, and spelling, or face the bloggish wrath of Mercenary Proofreader, who will mock you mercilessly and say harsh things about your book.