This guide is experimental. I tried this a few times and it didn't seemed that bad idea to me, but it is still in the testing phase. Try this and write me your results please. When it will be bad I will delete the build.

THIS GUIDE IS NOT FOR BEGINNERS!

As jungle Garen, you have

+ Higher chance of winning top ( Garen counters only Olaf)
+ Clear speed, dragons early (5)
+ With the buffed cinderhulk you are even more tanky/enables a bit o'damage too
- Certainly not the best damage
- Ganks

Well, Garen's ganks are not as good as they could be. I suggest playing with someone like Darius on top - you wouldn't need to gank so often. You don't sacrifice your top-winning potential which is better. Garen has good clear speed and dragon on lvl. 5-6 which is average or good. Enchantment: Cinderhulk makes him even more tanky then before. I think it is a good idea but I am still testing it.

Call your team on blue buff for help - this will help you a lot in early clearspeed and dragons. Continue your blue route - blue, frog, (gank optionable) wolves/raptors, recall, krugs, red, (gank) middle camps, recall, dragon solo, GANK or Counterjungle blue buff then gank. There your jungling phase almost ends. Be sure to clear some camps while wandering through jungle between ganks, especially top and mid (assassins will have a hard time getting you down and will waste their precious skills on you while your midlaner kills them. YOU ARE A TANK. YOU DYING - AS LONG AS THEY GET KILLED AFTERWARDS IT IS FINE.

Pretty much like normal Garen, only that you have Enchantment: Cinderhulk as first item instead of Sunfire Aegis. You certainly want to utilize your W so items such as Frozen Mallet aren't that great - I usually prefer Frozen Heart over Randuin's Omen as Garen.

Quite the best defensive runes on Garen. You can get your own if you want, I am not a runes pro.

You max E first because it is a good AoE with still enough damage. You have two choices then: you are enough tanky from items, your team is winning everywhere and you have positive KDA - go for Q earlier for more damage; the second option, obviously, is that your team is bad, enemy midlaner/ADC/jungler is fed or you are behind. Yeah, go for W, even though the resists bonus doesn't upgrade.

P: Health regen only. Quite good when combined with mastery:perseverance and Warmog's Armor.
Q: Your anticaster (silence) and second way of damage tool. Short range gapcloser. Stacks with AD.
W: Your defense skill. You get a shield (active) that stacks and +20% Armour and MR passively.
E: Your bread and butter. AoE damage, stacks with AD. Movement bonus while active is neglectible.
R: Killsecurer. Deals damage proportionate to enemy MISSING health (deals tons of damage to low-on-HP champs. Quite some range packed.