I've seen Leona being touted as UP by a lot of people, but personally I find she brings so much to the table that she's an incredibly viable pick already. I figured I'd share the build I use, as it's never failed me so far and I'm currently sitting on a nice 100% win ratio with her (only in normals, so not very impressive I know, but still)

For runes I go for magic pen marks (so that your abilities are slightly more threatening damage wise), armour per level seals and magic resist per level glyphs (this lets you itemise more towards health and regen, with any resists from items being a bonus). Quints I haven't finalised what I really like, but flat health regen helps her early game lane sustain out no end.

Standard tanky 0/21/9 masteries, making sure to pick up gold/10 to give a bit of a hand seeing how Leona's last hitting abilities aren't amazing.

These are up to personal preference really, but I love flash for both crazy initiation and the ability to jump out on low health so you can regen up and jump back in.

My second summoner spell switches around between ignite for catching cheeky enemies who actually manage to escape a bit, or CV/fortify depending on what the team needs.

Max W first because it gives you your insane tankiness and lets you vaguely clear minion waves. I make sure to get a point in each skill by level 3 so that you can use your full combo when it's needed. If you aren't being harassed much, Q can be used to make last hitting a bit easier as it resets attack timer and gives a bit of bonus damage on hit.

General skill priority is R>W>E>Q

Starting off with a regrowth and pot gives you good lane sustain, and sets you up with a quick philosophers stone to get your gold gen racking up. I generally stay out until I can pick up philos and boots, sometimes even get philos, boots and HoG all at the same time if you don't get harassed too much on lane or manage to pick up a first blood.

After that I almost always go Mercury Treads so that you can soak up CC's and magic like a baws. With your scaling magic resist runes, boots and your shield up, you should be looking at a huge chunk of magic resist even by the level 6 mark.

Next up comes the sunfire cape, though sometimes I throw in the phage for a frozen mallet as I'm getting this. Sunfire does for your armour what your merc treads did for your MR, as well as giving you a whole chunk of health and making farming a little easier from the passive.

Next up get or finish your Frozen Mallet. Some people prefer to go Trinity Force, but I don't think the damage you get out of it is worth the cost and with the amount of CC leona is chugging out with a Frozen Mallet, the move speed isn't going to be an issue. It's also a bigger, longer and more reliable slow. I'd rather have 100% chance to slow on hit than 25% and a bit more damage any day. Not to mention, matched with a sunfire cape you're looking at a smooth 1,150 bonus health to go with your sweet resists.

So now we have health, resists, a perma slow and aoe damage. Time to rack up the Health Regen for the ultimate tank pain. If the enemy team has lots of AP, hit up your Force of Nature first. If they're mostly AD, get your Randuin's Omen. Both give nice amounts of HP regen and Omen's passive and active is always welcome. If they are a fairly balanced team I'd generally go for the FoN first because it lets you keep racking up the gold from your HoG a while longer and gives you more health regen.

Finish up whatever item you didn't get before and you should basically be unkillable. The game should generally end by this point, but if it doesn't I like to throw the enemy team off a bit by getting a Madred's Bloodrazor. It's expensive for sure, and 90% of games you won't end up getting it, but the %health damage combined with your permanant slow and epic CC from abilities means you can rack up some good damage with it, and your magic pen runes will compliment it nicely. I also gives you a bit extra armour which never goes amiss, and the attack speed buff will let you ensure your Frozen Mallet procs even more consistently.

The Madred's is of course entirely optional and can be replaced by a whole bunch of things that are situation dependent. If you are actually getting hurt by people (unlikely) swap in a warmogs or banshees. If you feel like getting even more hp and slows you can throw a rylai's in there (though it's kind of a waste due to Mallet). Personally I quite like MBR, and people don't expect it, but if you're loath to build it for some reason, there are plenty of other options open to you.

I like taking Leona on a solo top lane, even though it doesn't make use of her passive much. With your regrowth/philo/HoG you become pretty immovable in lane unless you're playing overly aggressive. Generally let the opposing player push and just focus on your last hits early game, using shield to run in and get last hits and to weaken whole waves of minions for yourself while not taking much damage from whatever they try to throw at you.

If you've got a jungler who can put out some good damage then you should be able to secure a kill or two if they come and gank your lane if you manage to land your stun combo. Don't activate your Q until after your E has landed, so that the stun doesn't overlap with the root from E and waste some CC time. Every half second counts with your CC early game. Try to activate your shield just before you use E so that it 'splosions all over their face. Once you're 6, use your ulti with the center just behind them for when they inevitably try to run and you should hit your stun more often than not, or at least slow them massively.

Mid Game Leona can serve two roles. One role is to act as a roaming ganker, cracking skulls with your CC combo and on-hit slows once you hit Phage/Mallet and making sure nobody escapes from your carries. For this follow the same as you did early game, W into E, Q once you land, drop an ulti behind them.

Leona's ulti is deceptively long range, and you can stun/slow around corners and into forest quite a bit. The cooldown is fairly low so don't be too afraid to try and land a blind ulti if someone runs around a corner to escape.

The other role to use her for is an immovable object in a lane. She might be a slow pusher, but nobody really wants to gank leona and your slow farm and progress up a lane can bait team fights onto you quite well. The key to this is map awareness and communication with your team. If you extend too much and your team is on the other side of the map, you'll just die slowly and embarassingly. Make sure your team are within range to launch a counter-offensive and be aware of how many enemies might be coming for you.

By this point you should be a true tank, and the enemy team will struggle to take you down even with co-ordinated effort. Keep your flash on hand if they do focus you and you shouldn't get taken down often at all. If you do flash out and the enemy starts focusing other allies, harass with your ulti from afar, regen up a bit with your Philo/FoN/Randuin and jump right back in again.

Your job is to CC the enemy team into oblivion while peeling enemies off your carries. E and Q are amazing for picking people out who try to run past your team to get to a ranged carry at the back, and once you've hit them once your chain slows should stop them from reaching their target.

You might be a tank and you might be incredibly difficult to take down, but key to good tank play is not to be suicidal. Yes you could jump in to that group of 5 enemies, ulti them and get your team to gank them, but if your team isn't quite on the ball enough all it's going to end up as is you getting focused down and then a 5v4 fight against your team with no tank. It's far better to pick off stragglers and people on the edge of groups and focus them down quickly rather than jumping into the middle of a bunch.

Overall Leona is a true tank, bring a horrific amount of CC to the table and racking up some great natural resistances with her shield. You should be leading team fights from the front and getting stuck in, but always keep the awareness to get yourself out of the fray if you need to regen up. It's better to be out of a fight for 5 seconds and be able to leap in again than to die horribly in a fire and leave your team with no tank.