This is a build I made for Garen that I find it hard to go negative with even in ranked games. He's strong early on in the game and can even destroy and zone out a good Nasus when soloing top lane.
This build contains a lot of regen with a little added damage, it is a very expensive build so don't expect to get anything more than the Infinity Edge by the end of most games.

Dodge Runes that will work with Ninja Tabi and your masteries to give you a ton of dodge, something that never hurts.
Armor Pen, most all his attacks are Physical (Judgement, Decisive)
Cooldown reduction because hes got no mana to worry about
Fortitude Quints for early game beastage.

Get Judgement first it will tear up many of champions at level 1.
Max E first, Q second and W last, taking your R at 6, 11 and 16 as per usual.

I go 21/9/0 just because Garen is already a beastmode tank and he needs the bonus in the damage department.

Ninja Tabi for Dodge and cuz they are cheap and give armor.
Spirit Visage is Something u should rush to get so u can lane longer.
Warmogs for massive heath, Atma's will feed off this later.
Atma's Impaler - Nearly doubles your AD when you first get it. Added armor is a bonus.
Warmogs for insane HP & HP regen boost.
Warmogs again.
End Game buy Red Elixirs for added HP.

Ignite + Ult is a great way to get dirty kills.

Ghost works well in conjunction with decisive strike for a huge movement speed boost. Pop E skill to remove slow effects by 50% and spin away trololo.

To harass when laning solo-top, pop your Q for a quick burst of movement speed and an on hit effect silence. After landing the Q pop your E and spin on them as much as possible, if the enemy starts to run away, this is great news for you because you will be doing so much free unanswered damage to you. After doing this twice or so and making a character with minimal health regen low, you can zone them out from this point. As soon as you can start zoning out your opponent from last hits you are very hard to defeat from this point on.

Pros
- Garen is pro
- Spin to win
- Regen out the ***
- Huge health bar to scare ppl
- Ult is beast
- Great for premades since he's not the greatest at 1v1s but is beast in teamfights.
- Can Q and E out of almost any bad situation.
- Great initiator

Cons
- Low dmg early
- Will have trouble in heavy CC
- Can rarly win a 1v1, stick with teamates.

Jungle
Don't ever try to jungle until u get your Atma's. Simply Garen's damage output in this built is too low to be effective in the jungle.

Lane
Garen is tanky and if the champs you are playing against are melee or can't harass very well, feel free to stay in the lane and auto attack minions and try to pick up some last hits. If your opponents are squishy, forget about farming and try hanging out in the bushes for a gank when they forget about you.
If you want u can wait till all 6 or 7 minions bunch up then stand directly in between all of them and use your E, u can hit all of them at the same time maximizing your damage output.