Seeing as you play Malzahar, you probably like point and click abilities and making the enemy ADC hate you. Look no further, my friend, than Leona!
Lore & Personality
A strong, female rolemodel, who does not rely on skimpy clothing to be sexy? A warrior, trained by a fierce tribe, who is both compassionate and driven for her cause? Relentless aggression, combined with epic skins? Love barbecue? Leona has it all, my dear friend.
Born to fight as a warrior of the Rakkor, Leona believed that the true purpose of a warrior was to protect and defend. When she was put to the test, fighting her long-time childhood friend Pantheon, she refused, resulting in the leaders of her tribe to order her execution. That is where the Sun God himself stepped in and protected Leona, resulting in the Solari, their equivalent of priests, to declare her the Chosen One. Pretty damn badass, all in all.
Abilities
Your passive helps your teammates deal more damage to those pesky enemies. If you hit someone, they are tagged by Sunlight, and any damage dealt by your allies to a tagged target deals bonus magic damage.
Your Q increases your next basic attack's range, giving it bonus magic damage and stunning the enemy target. Walk up to a bitch, smack them in the face with your shield, and watch as they cry about you being 'broken' and 'unfun to play against'. Sound familiar, former Malzahar-main?
It also resets your auto attack timer, so it's great for clearing wards or bopping fools on their head.
Your W gives you bonus armor and magic resistance for 3 seconds, which is nice. It explodes after three seconds, dealing damage to any fool dumb enough to not walk away when they see you coming, and also those resistances stick around for 3 more seconds if you hit someone. Sayonara, Ziggs bot. No AP damage for you.
Your E, oh man. Throw it out in a line, deal damage to everyone you hit, and dash to the last person in line. This stuns them for 0.5 seconds, so if you combine it by activating your W early, throwing this baby out, hitting it, and then press Q, the enemy is going to have a bad time. That's because the enemy, if you don't have a braindead teammate around, is dead after that bad time.
Finally, your ultimate literally calls down THE POWER OF THE SUN and slows everyone in range by 80% for 1.5 seconds. Combine that with the W-E-Q combo, and you can fuck bitches up for 3 solid seconds. Morgana Q? Don't make me laugh.
Gameplay
You only need to remember three words: Get. Good. In.
As soon as you have your three abilities, you go in, and you don't stop. You create presence in lane. Aphromoo approves. You are tanky as fuck and that's how you build. Start with a Targon's Brace, get Banner of Command and/or Locket of the Iron Solari (this item literally belongs with Leona according to the lore). Mobility Boots are a must, because you want to roam and make everybody on the map miserable. Again, a concept you should be familiar with.
Also, engage. You can afford to go in and be aggressive, as long as your teammate isn't a monkey and actually wants to get fed. Take the barbecue skin to get that message across. You can soak damage when you get an item or two under your belt, and your team expects you to set up plays. You should feel right at home with the point-and-click ultimate, the stun and the feeling of being a playmaker.
Good luck!