Being Garen has a few advantages such as he can easily rush in an out of combat using judgement and decisive strike you should lane with the squishy targets.Courage is also good for poking in an out of combat.His ultimate is good for not getting your kills stolen after you get infinity edge you should be able to deal a good amount of damage to really squishy targets if your are getting tanked buy last Wisper in mid game at the end when you might need to tank turrets get some health and tank it a little.You should keep your almost frequently use your abiltys.

You should mainly go an attack first because you have good movement speed an damage. also on the targets that are the most threatening focus on them. If you a lot of high damage characters on your team you might want to tank a little more.

Garen is a very successful jungler.Try to get cooldown reduction buffs an movement speed.Use
Your abiltys almost frequently when your jungle.