I play a shit ton of top lane.
Mainly if you want to improve in top lane, you should focus on wave mechanics.
Understanding when to push lane, when to freeze/slowpush is critical.
Aggressive play
Most of the importance of "when" to playing aggressive or defensive comes from looking at the map. If you see the enemy jungler bottom side, you can play more aggressive( poke enemy laner, force them to recall/ TP to lane) and shove waves.
There are many ways to play aggressive, but you have to consider all factors that go into maximizing the best trades.
As Garen you have to think of: Can I QE this enemy? If I can't, I probably shouldn't go for this minion if they are playing aggro...[Pretty linear aha]. But also: Can I use my Keystone in this moment/ Does my enemy have Bone plating up... etc.
Your main goal is to maximize trades in order to force the enemy to recall and teleport back to lane, if they have ignite then your job is 100x easier because top lane is more about defensive wave mechanics anyway.
Defensive play
**Freezing: **
Now, freezing wave is when the enemy minions out number your own, however the enemy isn't creating enough pressure to shove into your tower, so you last hit minions, trying to keep the wave equalized in one position (while trying to keep your waves outnumbered).
This method is especially effective when you already have a gold lead( Ie: A kill or something significant) because it not only keeps the enemy laner from farming(as you would punish them if they tried to) , but also prevents the enemy jungler from ganking you effectively( As you would use this strategy while the minions are on YOUR side of the lane).
Slow pushing: Slow pushing a wave is best used to build a wave (Moreso seen in mid-lane) in order to gain** lane tempo.**
Lane tempo is used to gain roaming priority. This is because the enemy laner is going to be occupied with the wave while you go do something else (Take dragon, herald, roam, etc).
When you play top lane, you almost NEVER want to slow push unless one of 2 things are true:
You are going to ROAM.
You are going to DIVE. (with jungler unless you are a champion with an advantage/high kill pressure).
YouTube guides probably explain it way better sooo…
TL;DR: Look up Skill capped Guides on Youtube