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To lane against ranged, wait out the early game then go all-in at 6.
It's a sure kill if they get close enough for starter Q.
So hide in the brush and wait for them to greed tower plates.
Combo:
(W before CC) - Q - E - AA - ULT
You can use W later if the enemy damage comes later
W gives tenacity
extra AA gives a huge chance to kill before ULT

Combo:
Flash - AA - Q - E - Ignite - AA - ULT

Mix in Ignite for Phase Rush + Nimbus Cloak, which ensures that E deals max damage.
Stridebreaker is essential as it gives you attack speed + waveclear + slow and Q - E - Whip procs Phase Rush
You can also Stride during E.
It's vital in letting you catch up to ranged champs (2:45) gives more range to catch up. You can use vs Quinn when he's using E?! (2:50)
Ignite for Nimbus Cloak
While Flash is useful at making a play, Ghost is better against enemies with good escapes.
Since ranged champs are a free kill every time your summs are up.
It's important to farm and sustain with your Passive until lvl6.
You're strong as long as you can close the distance, so your runes, summs, and items revolve around exactly that.
If you're ahead, clear the wave and proxy farm. Dive with cannon wave.
Build Hullbreaker second. The extra stats and tower damage are great.
In teamfights, burst a target then run away.

Against melee, take Conqueror
For your secondaries, choose between Nimbus Cloak (speed) or Cosmic Insight (summs CDR)

5:40
You can excel at mind games. For example Jax uses E to block Garen's Q, so Garen starts off with E instead. pretends to auto, then E. Jax will believe that Garen won't start off with Q at lvl3. Once that's established, Garen waits until Jax is low then goes in with Q - Ignite - E - W.
Conqueror helps a lot with finding solo kills. If Renek has no E, so Ghost and full combo at lvl4-lvl5, but you have make sure that. U can use W for bait when they dive.

Clear the wave with Q (the cannon) - E - Whip
You can try dive with the cannon wave. Worst case, the enemy loses xp and you could potentially grab tower platings.

Obvisously, melee enemies are easier as they'll let you close distance.

Play around vision as your first priority is gap closure. You can use Oracle Lens for that.
What makes this Garen strong? It's the extra autos (AA)
Stride - AA - Q - E - AA - R

In teamfights, you try to burst the ADC with Flash - Q - Ignite - E - R
Try to Stride as many enemies as you can.

Garen is a good champion. (maybe)