Rule #1 - Practice the AA -> Q and the AA -> E animation cancel since it will increase your damage output.

Rule #2 - Never be afraid to fight, but always know your limit, your passive is very forgiving when you are fighting since you can heal back up faster than they can, you can engage more fights without being forced back and can even keep up with a laner that is winning all the trades against you due to the heal, you do not need to back with low HP as you can recover them while hiding in the brush or under your tower, but learn your limits as that often leads to plays that are way to risky and makes it easier for you to die because you overestimate yourself.

Rule #3 - Don't take any **** from anyone, don't let your opponent **** talk you for picking Garen nor your teammates, you know better than them.

Rule #4 - Never BM to much, Garen has two great BMs especially paired with a mastery symbol, never over do it with your one handed handstand or the best dance in the game because sometimes it gets you killed, on the other hand, it is great for tilting, making the enemies focus you instead of your carries, doing everything they can just to froce you into a fight with them, ignoring the rest of your team.

Rule #5 - Stay in motion, never stand still, when using your E try to get infront of them or go in circles around them to avoid as much skillshots as possible. when using the Q (not in the laning phase) try to use the speed boost to get next or infront of them since many enemies can get away with a well times ability just before you Q, since you will stand still for a second while using the Q they can escape, making it a litle predicatble, if you shorten the distance between you and the enemy you will have a better chance at chasing them down and you will get the most out of your Qs movement speed buff.

Garen is know as being the straight forward Juggernaut that can deal tons of damage, everyone agrees that that is his biggest flaw but that exact thinking is something you can abuse, not only in trading but also by getting kills you shouldn't have.
Everyone knows the Activate Q pretending to run away then flash onto the enemy, Q, E and in the Spin before the silence runs out R, the pure shock of these manuevers is enough to throw them off guard since they probably want to try and use a skill to kill you or dodge the damage which they can't,You can get many kill when you are about to die with 2-3 AAs worth of HP left, then a Q -> Flash -> E -> R can burst down an opponent that had about 40-50% HP left.

If you are vs a meele Champion in the laning phase try to trade by starting with an AA, animation cancelling it by instantly pushing a Q and followed by another AA then use the speed boost to get away from his Minions and retaliation, if he is nearly dead by then or there are only few to no minions left on his side you can also follow up with a E, using the movement speed from your Q and maybe Stormraiders Surge to spin infront of him so you can hit another 1-2 AA as he tries running past you.

This was just the beginning, PM me if you have any Questions and I will add them to this build, I will also go out and record some footage on games vs all the toplaners you will face.
This is only the beginning of the guide and I will add more in the coming days.