Garen can be played many ways and in this guide, I will show you all the ways.

The first build shows how to build tanky dps Garen with a heavy AD enemy team.

The second build shows how to build tanky dps Garen with a heave AP enemy team.

( I did not set masteries or runes because I use the same ones as the first build. Just use magic resist seals instead of armour. )

The third build shows how to build full AD/dps Garen.

For Garen #1 and #2 I use masteries 21-9-0 looking for damage dealing masteries in Attack and armor and magic resist in Defense.

For Garen #4 I go 21-0-9 looking for the same things in Attack, and for utility I just chose what seems right :)

For runes I go the same for all 3 Garens because even a tank Garen needs survivability. So I take armor penetration marks, magic resist OR armor seals, cooldown reduction glyphs ( they work great for Garen since he doesn't use mana. Ability spamming, activate. ), and flat health quintessences.

My priorities are R>E>Q>W. Obviously level up your ultimate at 6 11 and 16. First max Judgement because it can do a lot of damage and helps get you great early creep score (minions).
Next comes Decisive Strike. The movement speed is great when your team wants you to iniciate, it helps take down turrets, and you can catch up to enemies to confirm a kill.
Last is Courage for survivability. The armor and mr helps when your tanking a turret for kills or for escaping.

I always max judgement first because if you stand in a middle of minion waves right after maxing it and push E all the mages will die by the time the spell is over and the melee minions will be very low health.

Garen may not be able to carry a team to victory so don't hog all the kills. But you don't really need to worry about allies kill stealing with a heavy damage dealing Q and R ability, and just stick to your enemy with your E and the kill should be yours. A great tank for soloing top and a great iniciater to start team fights and a really good Baron tanker.