Garen is a fighter according to Riot but according to me Garen is a great.. No not great but awesome tank, he is not such a good snowballer but he is a good defender. What i mean with that is: He is able to control lanes with his E (Judgement) and his Q (Decisive Strike) + He is able to escape with his W (Courage) and flash if he is getting ganked or just needs to run away due to low health.

You should get Armor-, Magic Resist- and Physical Damage runes (Tier 3 on all of them) to garen because you are going full tank on him so you are going to need that extra damage the runes are able to give you.

You are going full tank on Garen, that means that you will buy no attack damage/ ability power what so ever to him. You buy the Giants Belt in the beginning to build up too Warmogs Armor, you buy the Warmogs Armor because it gives you more lane time. What i mean with lane time is that you are able to spend more time in the lane due to Garens passive (Perseverance) that gives you more HP regeneration the more HP you got and you buy the Warmogs Armor du to the HP regeneration bonus and of course the extra HP.

You start with the Q (Decisive Strike) because it gives you a speed buff for short period of time and it removes all slows + it silences the enemy that's struck by the attack.
You want to get your E (Judgement) maximized as soon as possible because that's the ability (except for your ultimate) that makes the most damage and it is the only ability that are able to give damage to more than one enemy, minion, champions or jungle creatures. You DO NOT! want to start with Judgement because you will get slowed by minions if you damage them with this ability and will due to the slow that's given to you not be able to chase down your enemies as easily and it's not as easy to finish them of with Judgement
You will notice that your W (Courage) will be of very much use when you escape from a failed gank or push. Why it's so good is because it gives you reduced crowd control effects and it gives you a shield that raises your magic resist and armor (You will mostly need this if you fail to manage your lane or feed).

You want the teleport to either escape, gank or get to your lane fast and get the element of surprise on any enemie.
you wan't the flsh to either chase down or escape from your enemies.

PROS:
- Your passive (Perseverance) gives you HP regeneration
- Your W (Courage) passively increases your armor and magic resist
- your Q (Decisive Strike) removes slows and are a great escape + it's easy too finish foes off
- Your R (Demacian Justice) makes more damage due to your enemies health (low health = more damage)
- Your E (Judgement) makes a lot of damage to more then one enemy
- Garen have no mana

CONS:
- Your E (Judgement) will make you move slower through minion waves or Monsters
- Your W (Courage) have a very long cooldown
- Your R (Demacian Justice) are very hard to use because it's to measure when you should use and not use it on enemies.
- Your passive (Perseverance) will not work if attacked by a: large monster, turret or Champion.
- Garens abilities have a very long cooldown

You should use your E (Judgement) to get minions and use your Q (decisive Strike) to finish the big minions of.