This is my first guide, so im aware i might leave a few things out.
Rule #1 You are a tank so dont try being a carry
Rule #2 Dont get killing blows if you know your carries can get them
Rule #3 Peel, Peel, Peel.. Leona is one of the best champions to use to keep ememy champions off of your carries

Marks, i take are Magic pen, i use them because they help do a little bit of damage early game for first Blood.
Seals, Health per Level, When you use these with Flat Quints you have enough health early game to ( tank ) alot of damage
Glyphs, Flat magic res, Since i pick up a Dorans Shield first this helps equal out me resistances and the extra health and health per 5 is amazing.
Quints, Flat Health, extra 78 Hp at level 1? yeah why not

I use the basic 9/21/0 build because of the Cool down Reduction from the offense tree and the tankiness from the defense tree

Dorans Shield First, its the best beginner tank item in the game in my opinion.

Second, you want to get your boots, it depends on the enemy team which boots you will buy, heavy AD you will get Ninja Tabi and of Course if its Heavy AP get Merc Treads. Third, Aegis Of The Legion, you will need to get this because as a tank you are there to help your team not yourself.

Fourth, Depending on how the game is going i will either get Sunfire Cape Or Force of Nature.

Fifth, Frozen Mallet, It is a very good tanky weapon, it will help greatly is those chasing fights since it does slow the ememy champion so much.
Its hard for to say which items to get when because every game is different, if you feel like getting a MR piece for a Armor piece that is your call, like i stated before its all about the game you are playing

You want to tank your stun first to be able to stop the enemy champion from advancing or retreating when you have the upper hand.
Second, Zenith Blade, It is an amazing initiator and will get you in close to stun your enemy.
Third, Eclipse, by far my favorite tank spell in the game, Loads of Magic resist and armor and also damages enemies around you.
Take Solar Flare Whenever possible.
Follow the skill guide i provided up top.

I always take flash because it will get you in and out of situations.
my secondary summoner spell is exhaust because you need to be able to take out the enemy carry in the middle of a team fight.

Leona Is one of the best tanks in the game if you use her correctly.
While laning with a friendly carry always try to let them get the killing blow because the gold for them is so much more important than it is to you. It is your job to ( peel ) of enemy champions from your teammates, what i mean by that is, stun of of them Zenith Blade the other to get them away from your teammate, she is the best at this, it will take some practice but once you get good at it people will be very pleased with your performance.
When initiating a fight use Eclipse, aim your Zenith Blade and hopefully it lands on an enemy and stun away, always let your laning partner know when you are going in so they can help ensure the kill.
Dont be Afraid to use your ultimate to stun fleeing enemies, it is one of the longest reaching stuns in the game, use it to your advantage any way you see possible.