Hi! I am a Leona main and would love to help you learn her. ^^
I have rune and mastery pages made specially for her.
Runes I go:
Greater Mark of Attack Damage x9
Greater Seal of Armor x9
Greater Glyph of Scaling Magic Resist x9
Greater Quintessence of Ability Power x1
Greater Quintessence of Health x1
Greater Quintessence of Attack Speed x1
Why this works:
The attack damage and attack speed help a lot during teamfights/fights. Since Leona cannot spam abilities, your main tool of damage is by autoattacking. Having extra damage on your auto attacks will help a lot and can secure a kill.
The little ability power that you receive gives your abilities that little magic damage needed, while the armor, magic resist and health helps keep you alive. 80% of the time you will be building armor (especially in this meta) so the scaling magic resist will end up giving you more than the normal magic resist ones.
I added a screenshot of my mastery page for Leona that you can copy.
As for builds, this is what I tend to get:
and the last item can be either
or
(Depending on what your team needs.)
Mobi boots are cheap and help you move around the map quicker to help teammates. Sightstone for vision, do not upgrade to Ruby unless you have nothing else to spend it on. Randiuns will help with crits and the active slow helps to escape/engage. Zekes is great for Leona atm, with the extra ability power, armor and the passive to help your teammates. Face of the Mountain is good for the active that you can help your carry survive. Locket is good for the AOE passive (if the enemy has a lot of AP) or you can build Mikaels for the active which can make or break a teamfight.
I use to build Frozen Heart on Leona but I exchanged that for Zekes because they nerfed Frozen Heart while making it more expensive plus the passive on Zekes is just so much better now with the ability power added.
If you need anymore tips and tricks, have questions or would like to watch me play Leona/or I can watch you, add me in game. ^^ Hope I helped!
EDIT: Forgot to add, 8/10 times I take
instead of
for kill pressure in lane. :)