Garen is a very balanced hero who haves speed, damage and life.
The two spells are combined both for killing and the flash to move out combat when getting focused.

Garen at low level is a real punk killer because his first skill cause a lot amount of damage, silences the oponent e gives him real speed. But at first level he already has a greater amount of HP.

Using the third skill to removes slow debuffs and end the oponent life when he thinks to run.
Use the first skill to start combat and to run if the thinks go wrong.
Use the second skill always when get focused, you need to preview stuns and locks situations to not get embaressed.

Garen at high level, the itens combination will give to garen a lot of damage but not a "tryndamere way", its close enough the 4k life of garen will settle the combat.

50% armor and spell reduction and a good speed will not let will be killed in final levels always use flash to move out when u get 1 k life, back to use your ultimate to help team and use your regeneration to get back quickly to combat, you can do that you are the HERO OF DEMACIA!