You'll need to get the paint off of the porch. Don't use pain removers, the residue may end up forcing stuff into the pores and messing with the slab. If it won't scrape off, you could try a pressure washer or rent a concrete scarcifier to remove it. Depending on the length, you may need expansion joints. You can do this with leaving a tile joint ungrouted and using a color matched caulk, or you can use some of the products from www.schluter.com
or other companies. You would also want to honor (line up) expansion joints in the sidewalk with your tile in the same manner. You'll want the porch sloped away from the house if it isn't already.