Can I just paint over my brick fireplace or do I need to prime it first?

You should wash the brick with a mild acid and then rinse. Priming on a bare surface is always a good idea. There are many masonary primers but I use Zinsser Bullseye 1-2-3 for almost everything. Sealer - Bulls Eye 1-2-3 Water-Base Sealer (http://www.zinsser.com/product_detail.asp?ProductID=11)