What do I need to do to defend a gym?

In order to defend a Gym, all you must do is leave a Pokemon at a Gym that your team is in possession of. If your team does not own a certain Gym, you must first defeat and remove all Pokemon from the Gym. After you do this, you can claim the Gym with one of your Pokemon and begin defending it. The longer your Pokemon defends a Gym, the more PokeCoins you will earn as your Defender's Bonus. You can earn a maximum of 50 PokeCoins as you Defender Bonus Daily, so you will need your Pokemon to defend a Gym for roughly seven hours to earn this.