Leona works well as tank because of her Eclipse spell. She has a few stuns, and is good in team fights.

Doran's Shield is a good starting item because of the extra health and health regen right at the start.

After you get Boots of Speed you should either buy Mercury's Treads or Ninja Tabi depending on who you are laning against and if the enemy team is mostly AP or AD.

Sunfire Cape works well with Eclipse to kill creeps.

If you got Ninja Tabi, and go with a Force of Nature first, if you got Mercury treads, then get a Glacial Shroud, and then Force of Nature.

A Frozen Heart will give you some of the extra armor you need, as well as slow nearby enemies attack speed, which is good for team fights.

Guardian Angel gives you some armor and magic resist, as well as letting you revive after you die. This is good for team fights, because if they kill you and start chasing your allies, you can revive, and stun them helping your ally flee, or maybe finish them off.

For the last item, it's entirely up to you. Warmog's Armor is good for all-around enemy team or you can go with Thornmail for teams with a lot of attack damage.

It's good to put one point in Zenith Blade at the start, that way you can zip to enemies and harass them.

Atleast a point should be put into Eclipse near the start so you can survive a little longer and maybe last hit some creeps.

After that you should alternate between Eclipse and Shield of Daybreak. The stuns can come in handy for helping your teammates escape or when an enemy is running away.

Your ultimate, Solar Flare, has good range and and cast pretty quickly. You can hide in a bush or behind a wall, use your ult, zip towards them with Zenith Blade, activate Eclipse and stun again as soon as possible with Shield of Daybreak, by the time the stun wears off, Eclipse should be able to hit the enemy. This is a good way to start a team gank or initiate a team fight.

Flash is a good choice for running away, or jumping towards an enemy if your Zenith can't quite reach them.

For your second spell you can either go with Teleport or Fortify.