Garen Runes explained
There's one set of runes for Garen on patch
11.8
that is the clear favorite for LoL players and pros alike. The best Garen Runes on the position 
Top are Precision as the primary path and
Resolve as the secondary path with a pick rate of 97%. Besides runes you should take a look on Garen Build and if you are interested in pro matches take a look at Garen Probuilds. If you want to discuss
Garen then you should visit the champion subreddit.
Primary path on Garen
Precision improves attack and sustained damage on Garen troughout the match. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Conqueror is the best keystone rune to run on
Garen. This keystone rune provides additional power in the later stages of a game. As soon as
Conqueror is fully charged, Garen deals additional damage and gains life steal. This is usually followed up by taking
Triumph,
Legend: Tenacity and
Last Stand.
Triumph restores on Garen a part of missing health and award bonus gold, if a champion is killed. Garen gains extra Tenacity with every additional stage of
Legend: Tenacity. With
Last Stand you deal more damage while you are at low health.
Secondary path on Garen
When it comes to your secondary path, most
Garen players prefer to go with
Resolve.
Resolve empowers the survivability of Garen and increases the tankiness. On the
Resolve path, the best runes for Garen are
Unflinching and
Bone Plating.
Unflinching inceases Garen tenacity based on your missing health.
Bone Plating make Garen overall tankier, by decreasing the amount of poke damage in the early game.