basic fighting -
q first to silent the opponent ,then e for additional damage and when he's silent end press the w for blocking he's skill attack. use the ult only for the final blow, the less life your opponent have the more damage it will give.

laneing phase-
dont be afraid to make a lot of trades and be aggressive. even if u're loosing those trades garen's passive will compensate that lake of health in 15 sec. so you're not really loosing.

in team fights -
garen can be a real bully and go straight to damage source of the opponent team. in most team fights garen should focus the apc and Neutralize him, and garen has ability to do so thnx to his q and his tankyness.

when pushing/split push -
dont be afraid to go deep into the enemy territory, even if you're alone. its really hard to catch and kill garen even for 4 champions(i'm saying this from experience). his "q" disables slows and with boots of swiftness garen have an enormous speed, and his health and armor help u take the damage. btw cause of garen's speed u can always return back quickly to the team fight after pushing.

boots of swiftness -
garen is a close range fighter so to compensate this u need an extra speedto catch ur enemy.

frozen mallet -
give u more health and ad and help u keep ur enemy real close and getting hits.

warmog's armor -
garen's w give u enough armor for early/mid game so health is more effective.

atma's impaler -
the 15% crit works well with garen because garen's e can give critical attacks.
armor is always good and because of garen's massive hp atma's give garen an additional 45~ ad points.

spirit visage -
the cooldown reduction help garen be more aggresive an use his skills more frequently.
400 hp give garen en extra 6 point of ad thnx to the atma's impaler and extra tankyness.
spirit visage passive works well with garen's passive and both passives eliminate the need to recall.

randuin's omen(against strong ad team) -
randuin give help garen to catch enemies thnx to the active.
70 armor is enough armor.
500 hp give garen ,thnx to the atma's impaler,an extra 7.5 ad .

infinity edge -
for extra dmg and the crit go along well with garen's e.

pros -
neutral tank.
ult gives an immediate damage.
very easy to play with.

cons-
super close range (a good teemo at top can **** you up).
low Crowd control.
not enough damage as fighter.

if u play against long range champion at top its better to max ur q first for additional speed.