To put it shortly, Garen's combo (GALEFORCE, Q, E, R) will do 9400 damage against 0 armor and 5600 damage against 100 armor.
(0 armor - 9398 damage, 25 armor - 8451 damage, 50 armor - 7504 damage, 75 armor - 6557 damage, 100 armor - 5610 damage)

GALEFORCE will give you a much needed dash to get the jump on your opponent. Movement speed boost is also an added bonus, very useful for late game roams and/or outplaying your enemy through movement.
STORMRAZOR will slow your opponent, making it more difficult to outmaneuver Garen's E. Also the added Energized attack of 120 bonus magic damage will help get you more gold out of the FIRST STRIKE rune.
RAPID FIRECANNON, just like STORMRAZOR will give you the Energized attack which will help get more gold out of the FIRST STRIKE rune. The added 7% movement speed boost is also useful.
INFINITY EDGE will make your Q and E do extreme amounts of damage. Late game, Garen's Q will easily do over 1000 damage when you have both STORMRAZOR and RAPID FIRECANNON energized attacks ready. Garen's E, can critically strike for 110% (75% Garen's E passive + 35% INFINITY EDGE unique pasive) bonus physical damage.

Garen's E is his main damage dealer, so definitely max E 1st every game. At level 18 and full build, Garen's E can deal more than 6700 damage against 0 armor, 3900 against 100 armor.

FIRST STRIKE will give you more than 300+ gold late game from a single kill. ~2000 gold across entire game.
HEXTECH FLASHTRAPTION will help with proxying, as you don't need to walk around an entire jungle wall.
HEXTECH FLASHTRAPTION will save you from a dire situation to escape.
HEXTECH FLASHTRAPTION will get you closer to an enemy for you to engage first, giving you FIRST STRIKE gold.
FUTURE'S MARKET will help you get item powerspikes just a tiny bit faster than your opponent.
DEMOLISH will help you get very important plating gold faster.
OVERGROWTH will make you a bit more tanky. Health of a bruiser, damage of an assasin.
COSMIC INSIGHT will lower the cooldown of GALEFORCE dash.