Like almost every other Champion you have to take Flash. It's a good way to escape and it saves you in the Early-game from ganks.
You are a Top-laner, so of course Teleport is the first Summoner Spell that comes into your mind. Use it in the Early-game to get to your lane faster or make a Teleport-gank. It's also pretty important to have the Late-game pressure.
If you think you can get a easy kill on your lane you can take Ignite. However, Teleport is much more better in the Late-game than Ignite, and a true Top-laner takes Teleport, because it's very selfish to take Ignite over Teleport.
After you got your combo out, the +4% attack speed are somewhat useful, and compared to +2% increased ability damage it's the much better choice.
You should never underestimate the +20 health all 30 seconds! It helps a lot if you are against a strong enemy laner.
Natural Talent really fits into this guide. You are AD, so the +10 extra AD are obviously good, and all your abilities have an AP scaling, so the +15 extra AP are also good for you.
Leona has lots of stuns (and a slow if you missed your ultimate), thats means you get the 2.5% increased damage almost always.
To restore 2 health every due to Recovery can also help in the Laning-phase. To take Unyielding wouldn't be a mistake, but I would more consider taking this mastery.
You want to deal damage, but you are still a tank. Reducing the taken damage is always a good thing.
If you got no shields or self healing like in this case, you should take the +45 health from Veteran's Scars.
To lower your Summoner Spells cooldown is a pretty big deal, especially for flash. If you get ganked you will most likely need flash, because Leona is quite unmobile besides Zenith Blade [E] which you can use if the enemy jungler is ganking from behind.
I gave it a lot of thought if I should take Legendary Guardian or Swiftness. I decided to take Legendary Guardian, because +12 armor and magic resist in Team-fights are very helpful. But taking Swiftness wouldn't be a big mistake.
Grasp of the Undying:
There's no doubt about taking Grasp of the Undying. It's pretty OP right now and almost every Top-laner takes it, and so do you.
Damaging spells afflict enemies with Sunlight for 3.5 seconds. When allied Champions deal damage to those targets, they consume the Sunlight debuff to deal additional magic damage.
Your passive ability Sunlight is almost useless for you. Because it only works with allied Champions, it's not important in the early-game, but in the late-game you can probably deal a bit more damage with your passive if there's a Team-fight.
Sield of Daybreak [Q]
The next basic attack deals an additional 40/70/100/130/160 (+0.3*AP) magic damage and stuns an enemy for 1.25 seconds.
Your Q ability Shield of Daybreak is very important for this guide, because you can reset your Auto-attack. You should max it second to deal more damage and lower the abilities cooldown from 11 to 7 seconds.
Gains 20/30/40/50/60 (+0.2) bonus Armor and 20/30/40/50/60 (+0.2) Magic Resist for 3 seconds. When the effect ends, nearby enemies struck take 60/110/160/210/260 (+0.4*AP) magic damage and Leona retains her bonus Armor and Magic Resist for 3 seconds.
Max your W ability Eclipse first for a better Wafe-clear and better trading potential. It's not a big part of your combo, but is still good to deal a bit more damage and gain some armor and magic resist.
Zenith Blade [E]
Strikes all enemies in a line dealing 60/100/140/180/220 (+0.4*AP) magic damage. The last enemy Champion struck will be briefly rooted and Leona will dash to them.
The E ability Zenith Blade is the last ability you have to max. When you max it the damage rises and the cooldown gets lower, but because it's only your Engage-tool and Gap-closer it's not that important if the cooldown is a bit higher. For more informations how you should use this ability look at the "Your Combo" section.
Solar Flare [R]
Calls down a radiant beam of solar energy dealing 150/250/350 (+0.8*AP) magic damage and slowing enemies by 80% for 1.5 seconds. Enemies in the center of the flare are stunned instead of slowed.
Like with every other Champion you want to skill your ultimate ability at level 6,11 and 16. You should always use this ability as your Opener. The cooldown of Solar Flare is luckily pretty low, especially at level three, so it should be always ready for your combo.
If your matchup is easy, take this stuff, play aggressive and try to take an early kill. However, I like more starting with Corrupting Potion.
When you are playing against a lane-bully or if your matchup is hard, take the Corrupting Potion. I like this start more, because it gives you a good sustain and having always three potions in the mid-game is also worth something.
This item is pretty important, it gives you the mana you need, some armor which is always good for a tank and the +20% cdr are also good. Besides that, almost every Top-laner builds it because it's pretty OP.
This Item is very important and you have to build it every game. It not only gives you the wave-clear you don't have besides Eclipse [W], you need the active to reset your Auto-attack.
This item is not a must-have, but the stats are very suitable.
Leona has no AD scalings and you are very auto-attack based, so the +25% base attack damage are better than normal AD stats. And of course as a Offtank you can need some health.
The Black cleaver gives you some armor penetration, health, AD and missing cdr. You need all those stats, and with your auto-attack-resets you can reduce the enemy armor pretty quickly.
If you are facing much AD, these Items are your options. The reason why you shouldn't buy a Sunfire Cape is, because you already got wafe-clear due to Titanic Hydra, the Iceborn Gauntlet and Eclypse [W]. A Thornmail is also a good way to get more armor and probably better than a Dead Man's Plate.
Because The Black Cleaver is part of my normal build and the Iceborn Gauntlet is a core-item, it's not worth buying it when you have already 40% cdr. But if you don't build black cleaver and need more Magic Resist besides the Maw of Malmortius it's a good item to build.
This is also part of my normal build, you need the magic resist and more AD is also good. The Item got nerfed, so it gives no longer attack speed, but that's absolutely not important for you, because you are working with auto-attack-resets.
If you already have the Maw of Maw of malmortius, an Iceborn Gauntlet and the Black Cleaver and need more magic resist, you should buy this instead of a spirit visage to not waste +10% cdr.
Build these if you need more magic resist besides the Maw of maw of malmortius or if they got much CC.
Build these if you need more Armor besides the Iceborn Gauntlet.
If the enemies got many skill-shots or many slows, these are worth buying. But I don't like building them, because the 25 armor or 25 magic resist are often important for you.
If you are full-build you should buy this. Extra Health is always good and you are part of the front-line, so the movement-speed for your allies is also nice.