If I use the "answer your own question" option, will my answer be marked as accepted?
If not, that seems rather useless...
You can mark it as the accepted answer.
It's not automatic though. There are scenarios where you might have multiple options to accomplish the same thing. You might prefer somebody else's answer (and mark that as accepted) but still want your answer available to other people.
In terms of reputation, you don't get the +2 for marking an answer to your question accepted, or the +15 for getting an accepted answer.