If grinding for the items you need in Pokemon Go is getting old, you can buy them in the Shop. Poke balls, incense, lucky eggs, incubators, lures, storage and bag upgrades need to be purchased with PokeCoin, though. You have the choice of spending real money to purchase PokeCoins, but if you're low on cash you can earn PokeCoin just by playing the game.
As of right now, there is only one way to earn PokeCoin; that may change as updates are added.
To get paid, you must head on over to your local gym that is run by your team. An easy way you can tell if it is run by your team is by looking at your avatar in the lower left-hand side of the screen. If the circle color around your avatar matches the color of the gym you're good to go.
Once there, click on the gym to enter it. Then, look at the lower left-hand side of the screen and click on the icon found there. The game will ask you to choose a Pokemon from your bag to guard the gym. Choose a high-level Pokemon by tapping on it and it will be assigned to the gym. Don't worry, you won't lose your Pokemon, you just won't be able to use it in battles until the gym is defeated.
Now, this will only work if your gym needs another protector. If it doesn't, you will need to train at the gym to raise its prestige. Once the gym's prestige reaches another level a new protector can be added.
Only 10 Pokemon can be assigned to a gym, according to Niantic, so you may not be able to assign a Pokemon to a gym, no matter how much you battle. The other option is to head to a rival gym and take it over. Once the gym has been beaten, you can assign a Pokemon to guard it.
Be quick, though. Once a gym goes neutral, other's will try to assign their Pokemon to the gym. If they assign Pokemon from an opposing team you will need to battle again before you can take over the gym and assign your Pokemon.
Once you get your Pokemon out in the world protecting gyms, it's time to collect some coin! Go to the Shop. In the upper right side of the screen you will see an icon that looks like a shield. Tap on it and the game will ask you if you're sure you want to use your Defender Bonus. Confirm and you will be given 10 PokeCoins and 500 stardust for each Pokemon you have assigned to a gym.
It would be nice if you could just run around assigning Pokemon to gyms while collecting loot after each assignment, but alas, it doesn't work that way. The Defender Bonus has a cool-down time of 21 hours, so you can only collect your coins once in that time period.
Since you can only assign one Pokemon per gym, you need to go to various gyms, place your Pokemon and quickly collect your PokeCoin before your Pokemon are defeated. To win battles so you can take over gyms, read how gym battles work in Pokemon Go, and about how Pokemon Go changed battles for the better, and then check out our Pokemon Go pro-tips by people who play too damn much.
If this seems like too much work, you can assign one Pokemon to a gym per day and still collect the bonus. The coins will add up. Plus, the extra stardust will help you level your Pokemon so that they can better defend your gym.