it being flat has nothing to to with it. lol. you can buy a cheap gun from harbor freight for like 30-50 bucks. if you have an air compressor go for it. I would advise going to the JY and getting some free parts to practice on.
google and youtube will be your friend.
and as for learning, you can spray rattle can paint into a spray gun (I have done it before as spray gun creates a better and more predictable spray pattern) or you can buy cheap cheap paint from any auto parts store.
there are some basic materials you will need to get started right. Prep-all (cleaner), paint thinner. (do NOT mix the two. ie dont clean your gun with prep-all, makes a mess), grey scuff pads, tape if there are parts you dont want painted (grilles, etc). some way to support the bumper and lip. usually hanging it from the ceiling using wire works. the garage must be heated. do not try to paint below 60ish. mixing cups are great, paint strainers and stir sticks if you buy paint that needs to be mixed (you might be able to get all three of those for free depending on the store you buy paint from)