-Garen is very strong in Toplane and can carry teamfights but if don't get the early it will be more difficult to him.

little mobility a lot of single-shot damage
easily kitable very resistant champion
not very versatile excellent regeneration
easy to play against do true damage

-Grasp of the undying is better for Garen, with it he deals more damages with q in the lane.

-Demolish is better if you have a favorable matchup, go push your lane and take the tower to take other objectives with your team.

-Pick second wind if you are against a poke matchup. But conditioning is better in late game with w's passive.

-Unflinching synergizes well with w's active, Revitalize is good if you have healers but unfliching still better.

-Celerity synergizes with your q because you gain move speed.

- Gathering storm seems better for Garen to deals damages in late game beacause generaly you bought tank items in late game.

-Put two points in your Q to poke enemy better.

-W reduces the damage done by 60% (and get 60% tenacity) 0.75 seconds after activation.

-Max your E, it deals more damages and scale good in early.

-Garen deals additional true damages on the criminal with auto attack and Judgment, but the totality of R is true damage on the criminal.

-If you use your R in enemy's flash incantation you will hit him.

-Black cleaver sinergizes ery well with your E, enemy's armor will be break to 50% if you hit 3 ticks of your E and staks the Black cleaver. When you are spining you got the bonus movement speed too.

-Yoummu's make you run faster to catch easily a carry or someone and it the lethality is very strong on Garen.

-Dead Man's Pate makes you tanky and your next auto attack deals 100 more magics damages so with youmuu's and your Q will put mid life a carry. Sunfire can replace this.

-Adaptative helmet get you 10% of cdr like that you have 40% cdr and tank magics damages.

-Sterak make a shield on you and give hp and ad. It can be replace by an item tank.

-When you farm play with vision beacause if you are been ganked by enemy's jungler and die
the lane is generaly lose so ask help.

-Try to poke if you can, proc your rune when you are in Q is better.

-Care to the enemy range and don't get caught beacause you have not really escapes without your flash.

-Garen can't engage so let your team do this but you can punish placement mistake easily and make you sure your team can follow up.

-In teamfight you can focus the criminal but generaly don't ult him, it's better to ult a tank over feed or the carry. Spin on them to win.

-When your team is taking dragon or nash zone the enemy if you are fed or tank the objective.

Thank you to read and sorry for your eyes :)