Alright, so here is my quick and simple guide to Garen. Top-lane, Tank guide. Yes, I infact tank with Garen and i find it extremely effective. The way i play Garen is different then most. I play garen around his ultiamte. His ultimate does not mitigate with his AD. His Q is enough harass. E is great for minion kills if your forced into playing passive, which I'm rarely never pushed back. His W is what makes him a decent tank. You can add me in-game OceanKaylore. I can play games with you, and you can also check out my match history.

This isn't a perfect guide, its more or less to show you how i build and play Garen. It's very effective and there has been many times in games where In a 5v1 situation (Me being that 1) I've had their entire team run away from me, because i was virtually un-killable. Once you get thornmail+warmogs you can easily tank turret after turret, and your passive will allow you to fall back, and not lose xp or cause your team to lose a team fight.

I either run Flash/Ignite or Ghost/Ignite.

Early game harass/last hitting. If you see that minion in the back that is about to die, and you WANT that cs, Simply Q, Run towards your enemy which will then run. Simply hit a minion with some health left that will give you that cs, and continue towards the lower-healthed minion and auto-attack. 2 cs+Enemy lost some of his. (Do this for 5 minutes, and it adds up)

First Blood/First Kill Combo.
Harass with Q prior to combo.
Once he is about half health, (You must be able to determine this yourself)

Q...E...R...Ignite.
Now, 90% of my first kills are while i turret dive. So your combo will look more like this..

Q..E..W..R..Ignite + Flash to avoid turret kill.

If he flashes away...
Q..Flash...E.W...Ignite.R

That simple.