Hi this my first build here on mobafire so I'll give it a try.

What do you have to know about Leona? Stun!

Health runes as Quintessence cause in my opinion Leona lacks health.
CD reduction runes so you can spam your skills and help your team.
And some Armor runes and magic resist runes.
Why more magic then armor?
Base stats leona:

You will have more armor when you tart lvling

This is the same for the masteries we go full defense and some offense for the cd and ability power and some magic penetration.

1. Regrowth Pendant and mana pot so we can stay longer in the lane.
2. Philopher's stone same reason as 1 + gold
3. Boots with cdr
4. 2 Giant bellts. Yes 2.
Why no armor or magic resist?
You have your Z that gives some nice extra armor and magic resist + In my opinion she lacks health.
5. Warmogs armor. More health :p
6. Health + some armor + a nice passive
7. Some magic resist you can change this with a Abyssal Scepter so you will do some more damage and help the ap carrie in your team.
8. Guardian angel Magic resist + armor + a crazy passive

Yes your Z first it give extra armor and magic resist max this first.
2nd your e. Why nice combo with your Z in early game use this combo on squichy targets.
Combo: Activate z then use you E on enemy champion and do some damage.
combo at lv 4 : Activate z then use you E on enemy champion and then stun them with your A and do some nice damage.

What is your job as Leona focus the squichy target that do alot off damage.
Example: Ashe you can easly get trought the enenmy team and stun her. While flashing trough their team you can use your R to stun and slow then so you team can start killing them.

This was my first build I hope you like it.
And if you don't say me why :p