Often when you have a jungler garen is the one to solo top, though he's early damage isnt that great he can still make a kill or two out of the 10 first minutes. Pick Judgement first just to be able to take multiple last hits at the same time. As second ability you should take Desicive Strike and always engage the enemy first with it, and use judgement after or when to get away. Judgement removes slows from garen and will make it possible to run away in an hard situation like being up against their solo top and their jungler. Too avoid all these encounters you should always ward abit farther down the river so that you see the jungler comming and so that you can be able too see if he goes straight on or if he goes in to the bush right after baron.
You can always buy 2 wards and put one in each of them, that makes you able to see if the jungler is in there but doesnt tell you that the jungler is comming just until last second and it's a waist of cash if you ask me.
So the pointers on playing garen is probably, 1. Always ward, 2. Play with care if you dont see that you have the upperhand and 3. Always go sunfire cape as you first fullbuilt item no-matter what.

In mid game you will probably be about level 9-12 and you will have gotten Boots of Swiftness, Sunfire and brutalizer. This gives you and good amount of health, damage and movement speed so that you have all the requirements to survive, do damage and to be able too run away. Garen has the good luck that he doesnt need any buffs. More than baron you should leave blue and red to the AD and AP Carries of the team. I play garen off-tank so I wait until the tank engages in an fight and then I normally go in with an desicive strike and right after i do judgement just to damage the enemy as much as i can. Garen's Desicive Strike can also be used against towers and when taking down a tower you should always use it and it doesnt matter if you go alone with minions or with team always use it too maximize the damage done to it, cuz taking down towers will give you the win if the other team doesnt counter attack on you or someone else in the team and takes down 2-3 of you teamates cuz then it will be pretty hard to defend seeing that there is 3 lanes and you are less than they are.

Now it is time to start using Garens ulti on the high HP champs that get damages severely in the teamfights, if garen is there with his ulti ready there will be no chance for a tank or bulky bruiser to escape with a small amount of hp. When taking down towers always use you descisive strike to improve your damage done to it.