Garen can clear very quickly. Start Q Decisive Strike and full clear putting the 2nd point into E Judgment at LVL 3 to speed up your clear. you should finish your clear around 3:20 ready for a scuttle crab fight with LVL 4. You want to be backing with around 1100g to buy Berserker's Greaves. This is a big power spike for Garen as the number of spins from Judgment scales his item attack speed, allowing him to gank and clear camps a lot more efficiently.

Garen is very fun and unstoppable when he is ahead how ever is weak in the current meta because so many champions build Blade of the ruined king which he cannot abuse.

Mid Lane Runes

Phase Rush
Phase Rush is good for two reasons. 1st it makes it easy for Garen to take short favourable trades and 2nd it allows him to stick to and run down enemies using E Judgment. However, this keystone does make Garen significantly weaker in team fights and extended duels than his Conqueror and Grasp of the Undying rune pages.

Conqueror
Conqueror is the classic keystone taken Garen this is mainly because Judgment applies 1 stack per spin so it stacks up very quickly at high attack speeds. Conqueror is the ideal rune to take on Garen if you are going to be front lining for your team or your versing a champion that forces extended trades such as Yone or Yasuo, Conqueror might just give you that little bit of extra dmg you need to win the all in.

Jungle Runes

Predator
Let me just say Predator is not the best keystone for Garen it is most alternatives are far superior, however Predator could be one of the most fun. Build full crit and pop Predator, Ghost and Chilling Smite and watch those unfortunate Adc's disappear with one Decisive Strike.