This is my first Guide. Garen is one of my mains, and this is how I play him.

Pros:
Beefy!
A good initiator
Lane sustainability
Good harassing
His Ultimate
Has a Dash, Slow, and Anti Slow.

Cons:
Not a true tank, do not take Garen to tank, unless your team is all beefy.
If you build all tanky, you will be ignored.
If they survive your burst, your damage output is weak.
Your Ultimate has a 1 second channel, so if they go in a bush, it wont go off and you will rage.

Ghost+Exhaust.

Ghost will make you ******ed fast. Yes that's an oxymoron.

Exhaust will keep them in your spin so you can murder them.

I never take any other spells but Teleport and Flash could have uses with him too.

These can be mixed up a bit, I generally just go with armor, mr, and some armor pen. Keeping you beefy, and keeping your damage up.

Take a point in Q first. 2 points in E, 1 point in W. After this, you max your spin, take your Ultimate when you can, and max W second. Maxing your Q last.

The early point in Q might help you get a first blood, auto attack --> Q --> auto attack = HALF of someone's hp at lvl 1. After a few levels, it is just used for the silence and initiation.

If they face check a bush, you auto ---> Q ---> Spin. I generally save my W for after my spin because they normally wont fight you until you are doing spinning. This skill sequence will provide you with awesome harassing.

I always start with a health crystal because it builds into heart of gold, it also gives you a nice chunk of hp, which helps you stay beefy, and it adds a little bit to your passive.

If you have to go back before you can afford your 2nd HoG, lvl 1 boots are fine, but to maximize money income it's best to get 2 before your boots. Your q should keep you quick enough in the early levels.

Everyone likes Sunfire, I personally feel Atma is a better item. It will bump your spin by more damage than a Sunfire could give you.. and your spin proc's faster than the cape. Additionally, it raises your q, and your auto attacks, and it's cheaper. You forfeit some hp, but the 2 HoG make up for that.

Force of Nature is a must on Garen, the boost in hp regen, with your passive will make it so you can stay out on the field.... forever. The speed boost is nice too. If you are having a great game, and their ap carries aren't doing well, I will sometimes get the BF sword right after buying my Negatron Cloak instead of finishing the Force of Nature.

The rest of the items you will rarely see, because the game should be over. They can be substituted for other things if need be, ie: Thornmail, etc.

In a team fight, you really want to silence their AP carry, or something with crazy disables like Amumu or Blitzcrank. After that you go straight for their Carry, AD, or AP, whichever is the one that is hurting your team this game. It is your job to take them out of the fight. Spin, attack, get them out of there. Ultimate them if you can finish them off, or if a team mate will finish them. If they run, don't chase, come back and support your team mates. Also, try to get an ultimate off if you are going down.... it does a huge chunk of damage so it might help even if you die.