Hi Nancy. Is your porch metal or wood? I would use a good primer and then a nice quality outdoor paint that works with whatever material your porch is made out of. Does that make sense? You could most likely use a roller and a brush or mask off the areas you don't want painted and use a sprayer. Good luck!
The outer side or under side?
I would clean it first and use a good primer for either metal or wood!
Here's how I painted my porch floors: https://thehoneycombhome.com/how-to-paint-a-porch-floor-with-concrete-paint/
Clean and make sure it's completely dry. IF it's pressure treated wood, it needs to be at least 6 months cured before painting. Prime with Kilz Original. Paint with exterior paint. You can spray, roll, or brush but I prefer brush.