Garen should be played as a Juggernaut.

Pros:
His Q is a silence and is on a short cooldown, and can be used to take somebody out of the fight.
His W is very strong as it reduces his damage taken
Garens Passive is very strong and regenerates his health quickly.
Garen is easy to play
Garen is easy to understand
Garen's Q is slow resistant, and stops you from being CC'd
Can clear waves early game
E damage Buffed
Cons:
Useless* ultimate passive
Easy to kite
Much stronger champions compared to him
Low mobility when Q is on CD
No hard CC
Isn't as useful compared to other toplaners (Gragas, Nautilus, Fiora, Renekton)

After playing games as Garen, here are some tips i can give you:

Your Q can stop Slows, use it to escape.

You can activate Q while you're in E animation to speed boost.

When you're going in for a trade, make sure to activate your W when the enemy is about to hurt you.

If low HP play safe and out of enemy damage range, and use your passive.

Your passive doesn't stop by minion damage.

At level 2 you can burst enemies with your Q and E combo

Once you play more Garen, you learn his strengths and weaknesses and when he is good and bad