all you have to remember is what stats are best on him, you want crit chance, movement speed, health. resistance, CDR and AD.
crit chance since both decisive strike and judgement can crit, thus allowing you to further increase their damage output by a factor of about 1.5, my normal build will end up with about 45-55% crit chance which is huge.
movement speed or flash just because all your abilities are short range and your also melee, you want to be able to keep up with your enemies and also being able to slow your opponent will help glue you to your enemy.
health and resistance because your in melee range and thus higher probability that you'll be focused on, especially if your doing massive damage with your crits on judgement. you want to be able to survive, though resistance you wont need too much since courage can increase it so small amounts can be worth more.
AD since your primary damage is scaling with AD and thus increasing your damage output and also increasing your threat level thus making you the focus target instead of your squishies. and of course since majority of your damage is physical armor pen will be a primary stat as well.
Cooldown reduction sine you have no other limitations on your abilities. so that your able to cast these abilities more frequently thus dishing out more damage.
things you dont really want is attack speed, life steal, ap, and any mana related items.
attack speed you wont want especially if your getting cooldown reduction because majority of your damage is gonna be coming out of your abilities and thus the amount of auto attacking you'll do is very minor and additional attack speed wont show much difference in most of your conflicts.
life steal is for the same reason, since the amount of auto attacks you'll do is very small, life steal wont show any difference much at all.
and AP and mana items should be obvious since theres very little AP scaling and garen has no mana.
my typical build is:
Boots (mostly swiftness or merc treds),
Atma's impaler(last to be upgraded but avarice blade at the beginning to help farm and get gold easier)
black cleaver (additional health, armor pen, and the shred works on most of garen's damage abilities also cooldown reduction),
Defensive item (omen, maw, or bulwark'),
Frozen Mallet (to keep your enemies close to you)
and the last item is infinity edge (massive damage, crit chance, and additional crit damage to further improve your damage)