Pros:

Great Tank
A lot of damage
Decent Farmer
Easy Kill Stealing skill (R)

Cons:

No stun or slow
No ranged abilities

Start off with Doran's Sheild for the health, armor, and health regen. Get your basic boots and then rush Sunfire cape. when finished with Sunfire cape, get Ionion Boots of Lucidity for the cooldown reduction. Garen uses no mana so it is very useful for him. After that, get Warmogs for the insane amount of Health it gives. Atma's Impaler synthesizes with Warmogs very well. Based on the enemy team, u decide whether to get Force of Nature first (if they have more magic damage) or Thornmail (if they have heavy DPS) Note: You will eventually sell your Doran's shield, but don't sell it too early.

Garen is a great initiator. Run in with your Decisive Strike(Q) then pop your Courage(W) as soon as they start attacking you. After silencing them with your Q Exhaust then Ignite, then quickly Judgement(E) and watch as their health goes away. As soon as they hit below 20-25% HP, use Demacian Justice(R) This should kill most champions with no problem. Note: Target the squishiest champion on the enemy team.

After getting Sunfire Cape, u can kill any minon wave with your Judgement(E) and Sunfire Cape passive alone. Get in the middle of all the minions Judgement(E) and watch as creeps become gold!!!

I like using Exhaust and Ignite for my summoner spells for the higher chance of First Blood. Some viable choices are Ghost and Cleanse to escape from ganks, but with the speed boost from your (Q) combined with the damage reduction from his Courage and Slow reduction of his Judgement, Garen can escape very easily from ganks.