Since she came out she has been my favourite character trumping every other champion. Although she is hard to master in the beginning it will all pay off later. She is hard to farm with though but most of your money is from assists you get and trust me, you get A LOT of assists. Some people go AP with her but I disagree with it since her scaling is so low. Q=30% of AP E and W=40% and R=80% or AP. She was made for one reason and one reason only. To be a TANK. So without further ado... onto the guide.

PROS
Stuns Stuns Stuns
Amazing skill that adds Armor and Magic Resistance
Slow
Great helper in chasing
Great cash from assists since you get so many


CONS
Slow movement speed
Little damage
Somewhat long cooldowns
Mana hungry early game

For Leona's skills since they're magic damage based.

For the extra armor early on and through out the game.

For the extra armor early on and though out the game.

For the extra health which really helps early on.

I go 0/21/9 and grab Resistance, Hardiness, Evasion, Defensive Master, Nimbleness, Hardened Skin, Veteran's Scars, and Tenacity in the Defensive tree while getting Haste, Perseverance, Awareness, and Greed.

Good Spells

To slow an enemy to run or catch up to them or to lower their damage output.
To escape or to catch up to your teammate of enemy.
For closing the gap quickly or to get away.
Got a stun or slow on you? Cleanse and get back to business.

OK Spells

Good for saving a turret in need.
A good support spell. See where an enemy jungler is starting or check who goes where at the start of the game.
For getting around places quickly or escaping a gank but only a major one at that.

Off Limits

Not much use at all besides a bit more health. Might as well get a healing support instead.
Your not going to need this spell since your skill's don't cost much.
For junglers. I wouldn't recommend it at all.
I've only seen 3 games where this spell was used throughout the 1000+ games I've played.
I wouldn't use it since you don't need kills only assists.
The cooldown is just too long for this spell and the result is not good enough for the cooldown.

CORE

I max Q first and W next. The reason is because of the stun to help your teammates and for the more armor and magic resist from W. I max E last because the only reason you need it is to catch up to enemies and get a point in R when ever I can.

I start off with boots, 2 health potions, and 1 mana potion. I'd rather take a lane with a partner then solo top because she doesn't have the damage to keep your enemies at bay. Try to farm a bit early on and don't get too aggressive until you are at level 3. But you absolutely have to farm enough for Heart of Gold or your gold intake will be slow early on since there isn't much fighting. Once you get Heart of Gold build Mercury's Treads and start building Guardian Angel. The reason I get Guardian Angel as my first full item is because it gives a little bit of armor and magic resist. It will set your defenses pretty even and give you a good amount of resists at early-mid game.
Once you get Guardian Angel you should see what is the biggest threat to you and your team. If it's an over fed Fiddle then Force of Nature. If it's a huge damage dealing Yi then you should get Sunfire Cape(since Leona uses the power of the sun it would make sense to get a Sunfire Cape :P). If they are very balanced then I would get both. By the time you finish one these 2 items you'll most likely be in late game or mid-late game depending on how many assists you've gotten or how many creeps you've killed.
In late game the last few slots are up to you to fill. If they are a heavy AD team then get a Thornmail or turn the Heart of Gold into a Randuin's Omen. Need more magic res? Buy a Banshee's Veil or a Quicksilver Sash. Quicksilver Sash is one of the most neglected items even though it costs very little and has the 3rd most magic res in the game. Not to mention it clears anything thats stopping you from tanking. Got a Warwick ult on you? Quicksilver Sash and help your team rape him with a few stuns. Need some health? Buy a Warmog's Armor or Frozen Mallet. The slow from Frozen Mallet will help your teammates catch up to anyone too far and gives you some attack damage as well as a good amount of health.
Remember your job is to initiate and help as much as you can before you have to retreat or die even though you probably wont get that low. Use your ult in a big crowd to stun them and help your teammates. I always stun the biggest carry with E then Q while my teammates gather around and take him/her out.

So that's my guide for Leona. She is a great tank that can help turn the tables in any fight. Once you master her she is like an unstoppable Mordekaiser. I always build the core items and after build situational. Please comment and rate! I really hope I helped and good luck with the Radiant Dawn!