Myoko was a class of four ships built for the IJN in the 1920's, and the first Japanese cruisers to exceed the limits of the Washington Naval Treaty. They were heavily armed with 5x2 20cm main guns, and 12 torpedo tubes.
After Tone, Myoko feels like a step backwards into the Furutaka. Your guns are no longer in a convenient cluster, and you must learn the ropes of turret control once more. Furthermore, you can now only carry three scouts in a round, less than a half of what you could field with the Tone. And all your defensive upgrades are now gone, gone, gone. Whether you decide to speed through the ship towards the faster and tankier (but weaker armed) Maya or stay and unlock Myoko's true potential, is up to you. But once you get the ropes of her, Myoko is a formidable ship.
With a slow base speed with no upgrades to either it or the defense level, compensated by very powerful fully upgraded main battery, means that Myoko is probably the closest to the battlecruiser class in the game. A full broadside will inflict very heavy damage to CLs, can sink DDs outright, and even surprise BBs. Your advantage is that as medium guns they are much more accurate than BB's batteries even at maximum range, and reload faster. Myoko has the highest DPM with the 20cm guns of all Japanese cruisers - only tier XI Mogami with the fast reload setup can exceed it, but at the cost of significantly reduced range. Furthermore, your secondary battery gains two additional guns from Tone's rather anemic two-per-side. Keep in mind that all secondary guns can fire directly in front and behind you! Use this to punish reckless chasers, and align your ship to bring all AA guns to bear on planes at once. Overall your secondary battery is much more useful now and worth upgrading, it can often make the difference in a fight, and is especially effective against DDs and subs. For the main battery, it's recommended to keep it in salvo mode, and make good use of the E key to put shots literally on a dime with the CA's good accuracy stats.
Since Myoko is slow, with an upgraded boiler you only have the option of one armor item to still stay ahead of BBs. Going with just a boiler and no armor is also viable, but be careful, as you do not have much endurance in a fight. High tier CLs can be very dangerous as they can wear you out with their ridiculously accurate and fast firing guns, and you're far too slow to catch them. Don't engage on their terms! Make use of teammates and bases to get them in your sights or at least break contact. For the rest, remember the lessons learned about CAs so far - stay out of the middle of the fight, keep BBs blind, and hit them when they're crippled or distracted. Myoko works very well for escorting BBs, as its guns are able to kill light ships very quickly and can also bring a surprising amount of support fire to the table against enemy capitals.