Not a whole lot to explain here. It's Garen. He's a champ that used to be OP (overpowered for you newbies out there) and now is much more balanced and filling the role the company wanted him to fill. Now that he's in that role, he needs a new build, because the old one just isn't going to cut it anymore. That being said, lets start off with:
RUNE EXPLANATION: I know people are gonna have questions about my runes, and I have answers. For marks, armor penetration really isn't necessary, and crit chance is definitely good for Garen. Armor Penetration are just what I prefer, personally. The HP/Lvl runes I chose to go for going off of Atma's better late game (another ~110 HP at lvl 18 is about another 2 AD, which is awesome for just runes). Glyphs are glyphs. They're the "personal choice" rune slot, and mine happens to be the flat CDR glyphs. Flat Magic Resist is really nice too, but the flat CDR can go on nearly any champion and be useful, so that's why I chose them. Flat HP Quints help you survive a bit better early game, and are really important for almost every character. 97 HP can be the difference between life and death in this game.
The build idea is to treat him as a carry, a squishy even (:O gasp) early game. Stay behind the creep line, and try to last hit minions as much as possible (CS is very important for Garen now with the latest patch).
Exhaust coupled with Ghost and a Ghost'ed teammate could get you First Blood before minions spawn. Try to lane with someone that has some sort of CC or power (Cho'Gath, Pantheon, Veigar, etc.) that way you two combined can overpower an exhausted foe (especially with Exhaust mastery). As Garen, you pretty much own the bush, so stay in there and come out for minion kills. People are scared of Garen still, and you need to use that to your advantage.
Once you get to level 6, you should be able to get another kill as long as someone has been harassing the other team slightly. Exhaust and Ghost should be back up, so use them along with your ult to get another/your first kill. Once you get that kill, you should be able to push your creep wave to their tower and be able to port back for buying. Letting your partner get solo xp makes them happy, so don't be super quick about getting back. Once you get back, you can defend a 2v1 lane until your partner gets back, so /hug your turret and make sure minions don't take your tower :P Remember that minion kills are very important
With Warmog's and runes/masteries you should be at ~4k HP at level 18, which gives you about 80 AD off of Atma's Impaler. That's plenty of AD for Impaler to be worth your money, so don't worry about getting more HP unless you feel it necessary. Once you have the first three items, you should be a killing machine. The recent change to Decisive Strike makes it an ideal move for the killing blow. Activate it first for the speed boost, then spin into your enemy. Once you're done spinning, you should auto attack the champion for the decisive strike damage, which should be quite a bit at this point in the game.
Late game is where you shine. No matter what items you decide to get after the core three, you should be able to initiate pretty well. Keep in mind that the armor and magic resist on this site does NOT include Garen's Courage passive, so you have to add an additional 25 Armor and MR to your total. You can still initiate like a champ (no pun intended), so get spinning. Pick a squishy and stick to them like glue. Decisive strike can silence their caster, so try and aim for them, using exhaust and ghost when necessary. Most of your spin+Decisive Strike should take down a caster late game. If not, there's always your ult. If you spin in and take out one, then ghost out, making it a 5v4 or even a 4v4 if you die, you've done good for your team. Your team not having Garen is much better than their team not having Annie (example given), so don't be afraid to be aggressive late game if you did decent early through mid. 2 or 3 kills is really all you need to make this build shine for you, so try and keep that in mind during the laning phase.
The first three items are the core items, or the items you'll need for this build to be successful. After that, go in the order that best describes your game:
Order 1: Getting shut down and killed often? Guardian Angel, Phantom Dancer, I.E.
Order 2: Stalemate between you and your foes? Normal Order
Order 3: DEMACIAAAAAAAAAA-ing them to oblivion? Infinity Edge, Phantom Dancer, Guardian Angel
Too much CC on the other team? Getting focused? Merc Treads instead of Swiftness works just fine. Otherwise, you could switch out one of your offensive items with Banshee's Veil. 400 HP from that gives you an additional 8 AD, thanks to Atma's. It's not a great boost, but it also doesn't make you less strong for buying out of the list. That's important.
Constructive comments are welcome. I'll write a more detailed description if you'd like, just threw this together for a friend :) Hope you enjoy it.