If you're like me, you're obsessed with having strong enough Pokemon that you can dominate every gym in your area. Which is pretty much the only free way to get poke coins. It PAYS to have some strong Pokemon on your roster!
One of the easiest ways I've found to get high CP - over 1k CP at least, is to focus on capturing Pidgeys. Once you hit level 15, which you should be able to do in your first few days of playing, you will start seeing Pidgeys in the wild that are over 250 CP.
Evolve your Pidgey into a Pidgeotto for only 12 candy. It's CP should have nearly doubled and be around 500 CP now.
Evolve Pidgeotto into a Pidgeot for 50 candy, which should have by now. Your final evolution Pidgeot should be close to or over 1000 CP. Power him/her up a few times and you'll have a brutal squawking fighting machine, ready to wreck every gym around you!
Stay safe and have fun Poke-ing!