What needs to be done to prepare for brick fireplace painting?

Have you considered that a better place to to look for an answer to that question, or at least one where you might find a faster answer, would be on the DIY or HG (oh God I can't remember the whole set of letters of the other channel!)? I know that's where I would look.

Ellensa, what kind of brick is it?

Anyway, you will want to ensure it's clean first.
Make sure all loose dirt, mortar , etc. is removed. I don't know what type of brick you have, so just some basic info.

Then apply a cost of primer with an acrylic base in it. That acrylic base will help seal that brick for painting and cover those stained area's. Oil is not necessary, the morter, cement , etc. is actually sealed with acrylic quite well. Just read the sealer on the back to ensure it's good for using on
Brick/mortar.

I would plan on using both a brush for the joints, and a very thick roller - 3/4".