So, you’ve caught some powerful rare Pokemon, or maybe you’ve levelled up your Pidgeot and Raticate until they’re tough and strong. Nice! But Pokemon GO is not just about catching ‘em all. Ready to battle some Pokemon? You’ll need to be level 5 before you can battle at a gym. You’ll be able to find gyms on your map in Pokemon GO, just like you find Pokestops. Gyms are red, blue or yellow, depending on whether Team Valor, Team Mystic or Team Instinct is currently controlling it. If you’re really lucky, you might find an unclaimed gym, and you can claim it for your team just by placing any of your Pokemon at full health inside the gym.
How to Win Over an Enemy Gym
If an enemy Pokemon is currently holding the gym, you’ll get a chance to fight for it to earn XP and to claim the gym for your team. Before you join a fight, remember to check the current Pokemon’s combat points and Pokemon type so you can pick the best Pokemon from your collection. Fans of the previous Pokemon games will already know whether to send a Poison-type against a Flying-type, but if this is your first foray into Pokemon battling, just watch for pop-ups telling you how effective your attacks were. In combat, attack by quickly tapping or swipe your Pokemon to the side to dodge enemy attacks. Keep your eyes on the meter below your Pokemon’s hit points, and once it’s full, long press on the screen, and have your Pokemon use its special attack. There may be more than one Pokemon holding the gym, and if so, you’ll have to defeat them all before you can claim this gym for your team. If you’re plotting a territory takeover, make sure you reserve one of your top Pokemon from the battle for gym control, since only a Pokemon at full health can be placed inside a gym. 
Support Your Team!
Gyms in your team's color are already controlled by your team, so here you can challenge the current occupant to a practice match. Friendly battles are a great way to train, since both Pokemon will gain XP, and the gym’s prestige will increase, too. If there’s a free spot at a friendly gym, you can drop off a Pokemon to stay and defend the gym from enemies. Just like when you claim a gym yourself, your Pokemon will battle any challengers and then return to you when it’s been defeated, in an injured state but with more XP. So go find a Pokemon GO gym and challenge the current champion!