This is an introduction!!! Ya, Leona is awesome tank, blah blah blah.

I like to go with Health for Quints and Marks, I get health marks because Leona really needs health, something that i consider her weakness is the amount of health she gets and having that extra health early game is awesome. For seals and glyphs i like to go magic resist and armor.

I like to get the extra experience and gold in the utility tree because if I don't... I'm always behind on levels, especially when I jump lanes to help on team fights. And then go straight for a full 21 defense tree.

I tried going 9 into the offense tree instead of the utility (for the reduced cooldowns and magic pen) but it doesnt seem to help as much as you would think. I feel that the utility trees extra experience, gold, health regen, and mana regen, help her a lot more in the long run. Helping her to level faster so she can jump lanes more without falling too behind in levels and gold.

Doran's shield:
I like to start with a Doran's shield, it gives good starting health, regen, and armor. I find I survive a lot more as Leona if I start with a Doran's shield.

Mercury Treads:
The next item to get is the Mercury Treads (speed boots), always good to move fast early game and give you that nice magic resist and tenacity. You can switch these out for Ninja Tabi, but only if there are no CC's on the opposite team and there entire team is AD.

Kindle Gem:
I like to get Kindle Gem next, cause it gives Leona health and cooldown reduction, 2 things that Leona needs desperately to tank. This gives her an awesome base health and CDR enough to let you build up your defenses even more.

Glacial Shroud -> Frozen Heart:
This will help reduce her cooldowns (get that stun off more often), give her much needed mana, and give a lot of armor.

Force of Nature:
This is a good magic resist item to get next.

Randuin's Omen:
This is a great item for its health, armor, get some more CDR, just a great item for Leona, and the active is pretty sweet.

I would switch Randuin's Omen for a Banshee's Veil if the enemy team is heavy magic DPS.

Shurelya's Reverie:
Upgrade the Kindle gem to a Shurelya's Reverie. This gives another nice active for catching enemy champions and more much needed health.

Guardian Angel:
I would sell the Dorans shield item at some point after you get Force of Nature. But ONLY if you can rush Warden's Mail (builds into Randuins Omen) by selling it, or rush any of these final items. If you haven't sold it yet, sell it now to build a Guardian Angel.

I like to upgrade Shield of Daybreak first. Cause everytime it is leveled it reduces its cooldown. And makes it that much easier to CC in team fights early game. I next get Eclipse up next as it adds to armor and magic resist and is pretty sweet mid game. Lastly I level Zenith Blade cause it doesnt seem to do that great of CC or damage, good for end game catching though.

Shield of Daybreak:
This is Leona's main ability to control the field of battle. Keep stunning those enemy champions, use it to interrupt fiddlesticks crowstorm or nunu ult. Just a great CC. Can use this in combo with Zenith Blade.

Eclipse:
Eclipse gives Leona more armor and magic resist and does aoe damage to all surrounding units. Necessary to have on in a team fight. Also activates her passive Sunlight on all surrounding enemy champions, giving your allied champions more damage.

Zenith Blade:
Use this to initiate team fights, to catch running away champions, and sometimes to even run away.

Solar Flare:
Leona's ult is amazing. I usually try to wait till all the enemy champions are group together before i use it. So i can hit all of them. But if i need to keep an insane dps out of commision, I will use this and Shield of Daybreak to keep them contained. Also good for catching running away champions since it has amazing range.

Combo:
Eclipse -> Zenith Blade -> Shield of Daybreak -> Solar Flare
This seems like a great combo, that way your Eclipse goes off right when your next to them stunning them. Only problem i find with this is if you miss with your Zenith Blade, you have just blown 2 cooldowns... and have to wait for them to come back up. Not as good early game.

Zenith Blade -> Shield of Day Break -> Eclipse -> Solar Flare:
A safer combo, in case you miss with Zenith blade

Teleport:
Honestly the Summoner Spell I always love to get with her is Teleport. Mainly because as a tank your team will constantly want you for team fights or ganks. Its really nice to be able to teleport around the map to respond to these threats without having to walk everywhere and losing the experience. It's also a great counter to backdooring.

Clarity:
I go a lot of games playing with Clarity on Leona. Early game it is a amazing, you can harass and still stay in the lane with lots of mana. Late game its not needed as much cause you have Frozen heart which will give you all the mana you need. So its debatable whether its worth it or not, but i usually always like to take it. A tank with no mana cant really do **** to help the team.

Exhaust:
I really love exhaust, keeps enemy champions from running away, and reduces the damage they do. Helping you as a tank protect your fellow teammates. While your waiting for your other Cooldowns to refresh you can hit them with the exhaust and wait for you stun to come back up.

Flash:
I would consider getting flash next, always a good spell to run away or catch up to someone running away. But Leona's ultimates range is really large so you can always catch people running away with that, or a gank chasing you.

Just remember your job is to tank, not chase down champions and get kills. Remember to protect your allies. Sometimes its easy to forget. Just try to keep their best dps stunned and out of commission until your team can kill him. And remember your passive (Sunlight), gives extra damage to your teammates when they damage the target. Leona is always better in a lane with another player, rather than solo because of this passive.