Leona is the only and well female tank to the game. She has high cds, but does have some quite nice ccs and damage reductions making her a powerful tank. She can also be used as a support with her multiple crowd controls to shut down the enemy carry. My experience comes from 5v5s where I could easily go into battle soak up damage and regenerate full hp within seconds. This is my first online build so don't be too harsh.

Marks: I get magic penetration for the strong early game laneing stage. This helps increase the damage on eclipse. Same in the support sector because these runes will give you an extra early game poke.

Seals: Because I tend to get Force of Nature first, I get armor seals to bolster my lack in armor. This does depend on the enemy team composition if they're heavy ad or heavy ap.

Glyphs: Leona has relatively high cool downs on her skills thus having increased cool down over time will definitely help with getting the ultimate back before the next team battle along with greater usage on the other skills. This also allows you to skip out on cool down boots for the tenacity boots.

Quints: Use hp quints to give yourself that extra early game buffness or you can take some movement speed quints.

As clearly stated earlier, I said that Leona is a tank that can easily absorb damage. In a 5v5 match where you try to absorb 5 enemy champions while your team does damage, it is quite helpful to have 0-21-9 masteries to absorb the most damage possible both early and late game.

Starting off with the regrowth pendant allows for longer laning time as well as better harassing abilities without the worry about having to return to base. If you notice you're against an ad champion up top lane then go for cloth armor and 5 hp potions. After the first return to base you pick up Philosopher's Stone for the increased mana and hp regen not to mention the gold bonus too. The Mercury Treads are there for reducing enemy stun times on you so that you may cast your own stuns. A helpful item to have when under heavy fire.

As the game moves on you'll have to make a choice between getting that first negatron cape or getting that armor. If they're a heavy ap team and you're suffering a bit, go ahead and get force of nature before even bothering to get heart of gold. If you're doing fairly well in the game and under no real threat, take the heart of gold for increased gold income.

On the other hand, if they're a heavy ad team, I would rush Thornmail (if really heavy) or just Frozen Heart. These items both give high armor as wells as extremely helpful special bonus passives. If you want a more aggressive Leona take sunfire cape.

Nearing the end of the game you can finish it off with a force of nature or guardians angel based on the game play. A force of nature will ensure your survival longer and allow you to catch up to those last few champions that managed to sneak away.

Anything after this is mostly opinion based on whether you require ap against a weaker team, or more defense or magic resist against a damaging team. The build above is generally what I follow in game.

I start off with Sheild of Daybreak for the stun. Then I take Eclipse for the damage intake reduction. At level 3 it's nice to have a gap closer for the combo: (e w q) E to get close, W to reduce damage and deal damage later. Q to stun and make sure the Eclipse hits.

Eclipse will be your major damage intake reduction and thus is important for early mastering of the skill not to mention the nice stacking on damage output.

Zenith Blade will be your 2nd source of damage and thus master it second.

Shield of Daybreak gives less damage per point added thus master it last since the stun doesn't increase either.

For the skill sequence in game during fighting use Eclipse before your Zenith Blade. If you hit and start flying towards the enemy champion use Shield of Daybreak to stun them in order to guarantee the Eclipse will hit him. If you miss don't bother using Daybreak with the extra wasted mana. Your ultimate should be used upon initiating a team fight after your basic previous sequence as shown above. This should guarantee stuns on a few champions to allow for your carries to pick off the weaker ones fast.

Take: Exhaust is a must have in the top lane especially if you're going against that strong ad bruiser up top. Teleport is great for 5v5 games as it allows you to move around the map easily especially when the tank is needed to initiate a team fight.
Flash for fast escaping though I personally am a flash fan. As support I would recommend you take heal if the ad carry does not or clairvoyance.

Acceptable replacements: Ghost, Fortify, Cleanse. Ghost can be helpful in getting to team fights and out of them when necessary but really you shouldnt be dying that fast. Fortify will help in making sure towers stay alive. Cleanse will you get out of tight squeezes a long side of your Eclipse.

DON'T: Clarity, Rally, Heal, Ignite, Revive. Although they may have benefits, they don't give as much as the others. Leona has close to no mana problems as long as you don't wastefully spam skills. Rally is unused. Ignite save it for the damage dealers. Revive don't die.