This is my favorite way to play Garen so I thought Id make a guide. Im pretty new at making guides so dont expect too much, but I hope it helps people because well I haven't seen many builds like this one (not that I really looked Im sure they are there I only checked a few of the top rated ones). I normally use this build if we already have a tank but still need an offtank or if the enemy team has more than one tank. This is not an ad carry build! You will do damage but dont expect awesome nuke damage. The goal of this build is to be able to initiate a team fight and not only survive but usually get a kill. This being said Ill start the guide.
P.S. This is a guide not a rule book feel free to change or tweak as you see fit!

For my runes it took me awhile to find ones I really liked on garen. At first I used normal tank runes armor, mr, and health pretty much but I found that this works much better. You get a nice health boost, armor pen for damage, cdr since you rely on his abilities, and move speed to help chase (I used to get boots of swiftness but you just cant go without mercs most games so this was a way to compensate the drop in speed)

Nothing new here. For my masteries I chose a pretty straight forward tank build since this is for an offtank anyway.

Alright now for the items. I start with Dorans shield for the early survivalbility. From there I normally stay in lane until I have enough money to get a giants belt and at least Lvl 1 boots(assuming you are solo laning you might need to go back earlier and just get boots/wards if youre having to babysit a carry/any nonsupport lane partner and not steal creaps from them). As soon as I can though I finish my boots and get a warmogs. After that the build gets situational. Like if your team needs more damage go ahead and get the impaler/sunfire (these items are pretty expensive though so if you do decide to get them first you may want to consider picking up a negatron cloak along the way since you dont have much mr early), if you need the mr early get FoN, or if they are heavy ad (trynd, cait, talon etc) get a thornmail (thornmail would replace frozen mallet in the build). But for a balanced team the above build usually works really well. Where it lacks in damage/armor/mr it makes up for in health.

First point in Q for silence/speedboost/and does good damage. After that focus Judgement then courage next for the decreased cd and extra damage reduction, then grabbing Q last. (and of course geting ur ult at 6 11 16)

Not really based on anything other than personal preference.

This should be a no brainer for most players but for those of you that are new to garen here are some tips that help me.
--Last hit minions to help not to overextend. So dont 'spin to win' creep farm unless you need to thin the crowd so as not to get over-run or push all the way to your turret (since its hard to last hit with turret attacking)plus if you only last hitting it saves you cd on judgment for enemy champions.
--When playing as garen you can really change the pace of a match early on. Being aggressive early game can help you get an upper hand in creep score and lvls if you keeping your enemies scared of you. That being said dont jump in there and be stupid. Know your limitations if an enemy champion is close run up and silence him and spin then W as you back out. You shouldnt have any problem dealing out more damage then you take this way and the damage you take your passive heals if you just stay back for a second.
--Make sure your silence hits BEFORE you spin. If you dont silence them you are wasting the damage on your Q plus they can still hit/cc you when you spinning.
--When using your ult never use it to take a kill from one of your carries. If you are tank/offtank you do not need the kills let your carries have them. Only use it if it will save someone on your team/ you are soloing someone/ someone will likely get away or is fed and needs to die fast
--Also never open with your ult....I know seems like common sense but Ive seen people do it. As a general rule of thumb you should really only be using your ult if someone is at least below half health I generally wait till they are about 1/4 health though
---Hmmm I guess that about sums it up this is my guide I hope its at least half descent Im making this at like 6 in the morning so if there are any mistakes just ignore it or leave a comment for me to change. =D