Keep in mind that Garen was a great lane bully before the Juggernaut patch. He used to be able to bully people even in unfavorable matchups, but that's not really the case anymore. He had a fair amount of power shifted from his early game into his late game. He still does great in favorable matchups, but you have to play somewhat safe otherwise.
Personally I try to Q my opponent a few times right off the bat. It doesn't do a whole lot, because even if you trade well you're just getting a potion or two out of them, but it shows you what kind of damage you'll be doing. Unless you use ignite you're not going for kills just yet, even if they get low, you're just trying to get your opponent low enough that they will willingly give up control of the lane and let you freeze the minions.
If they make a mistake feel free to jump them, but until you know him better it's probably best to let your opponent come to you. Garen is fantastic at punishing mistakes after all, and once you get a few items on your opponent getting near you is a mistake. Just know your limits, and don't chase too far unless your allies are pressuring the map heavily elsewhere.
If you prefer videos look up stonewall008 on youtube. He doesn't do guides, but his Garen videos should offer a bit of helpful advice.