Before I start, I would like to say sth. about the summoner spells! U take smite and that other blue ****(which gives mana back, Garen is very manahungry, u must take it!). Always go blind pick with Garen and pick in the last few seconds (before Champion Pick ends) Smite. You arent the jungler, but Smite is such an important thing on Garen

Laning Phase: Always help your jungler to clear faster. Start with your ultimate and try to last hit the Gromp/Golems. Them, go and smite all his buffs away! If u do it correctly, ur jungler should be lv 1 , while u have already reached lv 6. GJ, succesfull laning phase!

Once u have 5 Tears, go aggrrssive. Ult the enemy as soon as possible. Ur ult has at lv 6 only a cooldown of 6 seconds! Use it to CS all the minions away (especially from ur ADC) If u have 200 CS, while ur teammates do have less than 50, u will have a great late!(But dont forget to help ur jungler to clear faster!

Once u have 5 pieces of that crystal, u have actually won the game. If u are able to land ur ult correctly (its an high skill cap skillshot), try to KS everything u can! But remember to help ur jungler. Tell the enemy team his position all the time and smite/ult his buffs away! If u do it correctly, ur jungler should now be lv 6-10, while u are lv 18. if it is like that, tell the enemy team to report ur jungler for unskilled player and hope that he gets a perma ban, huhihahuhihahuhihahuhiahuhiha

Garen is the hardest (high skill cap) Champ in whole LoL. His ult is very hard to land ><.
Its not like Lee Sin 's Q which flies like that -> (Lee here)

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>(enemy here)
His Ult is actually like (Garen here)---|
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> (enemy here)