Hi and welcome to my (not so much) indepth guide for tanking with Garen on Twisted Treeline!
I have been getting pretty good results with this guide that I hope you will get :)

Pros:
- Almost impossible to kill in team-battles.
- Great in team-battles.
- Mages and AP champs are worthless against Tank Garen.
- Still with this little AD he can still damage alot.
Cons:
- Vulnerable solo on lane and against CC champs.
- The enemy team will target you first in team-battles.
- Because of his high armor, enemys with high armor penetration will kill you faster.

I choose Flash and Exhaust because they are the best summoner spells you can take as a tank.
Flash to escape a fight or secure a kill,
Exhaust on the other hand is a great skill for ganking and targeting in team-fights.

You can really remove and add items in this build by need, but I think this build of items is the best for tanking in Twisted Treeline. You will absorb a lot of damage and deal a lot of damage with Sunfire Cape + Thornmail. And the little AD boost is great mid - late game to damage a lot more.

There is a few things you should think of playing as tank garen:

-The first and most important thing is to always go with someone in a lane! This is very important because otherwise you will get killed far to many times.

-Try to lane with an Assassin, like Nocturne, Master Yi and Twisted Fate. Most of the assassins are squishy so that in combo with tank garen = win!

-If laning with a squishy champion(Which you should)take the damage and try to prevent the assassin getting hurt.

-And one last thing, with your exhaust you should always get first blood with this tactic:
Run to the middle bush in the bot lane first. As soon as another champions arrives, you lay exhaust on the champion and the assassin in your team should jump forward and the rest of the team attack him.