Garen is a beast of a tank and highly under rated on his tanking ability.

Pros

As Garen most people expect you to be an AD DPS
Becomes quite god like late game
Can still get kills and harass very well
The silence can take out there AP carry s or Supports and can screw over there team composition

Cons

People Will flame you and say garen cant tank and hes a useless champ
can be harassed easily early game

Personally I don't use runes as I prefer to buy champs but these would be the runes to grab to maximize your damage as a tank and still be able to charge through and come out on top.

i take a 0/21/9 as you gain the ability to tank and have the extra experience, the three points put into perseverance is not wasted as its Regen works really wow with Force of Nature and his passive. If you like using Ghost then i recommend you change the gold per 5secs and putting it into the improved ghost not my personal favorite but an option for those garen speed abuse players.

With the items they're set up for tanking heavy AD champs and can be easily built to withstand AP teams. You can replace Randuin's Omen with a banshee's veil. Although you grab warmogs you think that madreds blood razor counters it, but its surprising to see that no one gets it when they vs a garen. You could replace the ninja tabi with mercury's treads or boots of swiftness but when its paired up with the dodge mastery's and dodge runes it can turn you into a unmovable wall early game and late game. Getting stacks on the warmogs is no problem as he can farm like a boss.

With Summoners spells you can grab others such as exhaust and ghost for those people that like to run real fast and slow down those stragglers. I chose ignite and Flash because I find it more useful for running away and disabling healers and insane Regen (mundo).

For using his skills correctly as tank I normally engage with Q ready and silencing there biggest caster threat such as there AP carry, support or there most skill dependent champ mean while your amongst the commotion pop your W and then start spinning and dropping them down if you have them on the run I advise that you ult there carry in a team fight even if you might be kill stealing from a carry on your team. The way I see it is the sooner there carry is out of the fight the quicker your team wins and the more health your team has for a push after the initial fight just use your cool downs of your skills as they pop back up. If by chance your team starts losing the fight you can just flash Q W and when slowed spin and you will be uncatchable.

Overall Garen is my personal favorite champ and very under rated when it comes to tanking.