I wouldn't recommend playing Garen first time in ranked, but if you are forced into it, then use this guide. You can read this if you are panicking in the loading screen trying to figure out how to play Garen.
I personally never read the guides with all the lore and that rubbish, this is about gaining elo fast, this means playing 1 champion, and I recommend Garen.
Garen beats most common tops, not including some, like Yorick for instance, so never pick Garen against someone who you know counters you, but most AD casters, tanks, etc. will get shredded.
I used Garen and Vladimir to get out of elo hell, but I think Garen is definitely the easiest way to gain elo, but I do recommend having a secondary lane, mid for instance, incase somebody else takes your lane.
As you may already know, playing only 1 or 2 champions in ranked is definitely a good way to get elo, because after a while you start to learn all the counters, all the play styles, your role, and just about everything needed to know about that champion, I'm sure there are others out there who have played Garen for longer than me, but this is what I have learned from my games as him.

The HP regen works great on Garen, because if you are getting harassed just play a bit more defensive, and the combination of the HP Beads, a HP pot, and Garen's passive, will heal you up fast.
If you are versing a harder lane like Yorick, Teemo, Vladimir or somebody with very strong harass, I recommend spending all your money on pots, rather than saving that extra 80g, also, if you are versing a very strong jungler and you think you will be winning your lane, then buy an extra ward, so you can stay in lane longer, and you will be able to play more aggressive in the knowledge that you won't be killed by a Lee Sin or Nocturne...

There are some champions that you just cannot win against no matter what, and there are some which will be hard, in this chapter I will list them, sorry if I miss some, I am open to adding more:

Do not play against:
1 Teemo
2 Yorick
3 Jayce
4 Pantheon
5 Olaf

If you pick before them, try to switch lanes with Mid, or ask for very frequent ganks.


Be cautious around:
1 Elise
Try and dodge her stun, harass her with Q as much as possible, she has little sustain.
2 Vladimir
Make sure to silence him if you think he is going to pool, do not try and harass him, as he will heal it all back.
3 Darius
Darius doesn't exactly counter Garen, but his DoT is painful, and stops Garen's passive, and his ult is better than yours.

You will demolish:
1 Riven
Silence her during combo, and you will reek havoc.
2 Mordekaiser
He is rarely played now in ranked, but if you ever verse him, you will be LEGENDARY!
3 Wukong
Wukong is very common, and Garen is a great counter pick for him.
4 Renenkton
He is another typical AD Caster, a Silence after his first dash will stop his combo.

Don't use your ward unless you are pushing, that is unless they have a lvl 2 gank champion, or you feel vulnerable, League is about pyschology, and knowing you are safe will make you play a lot better.
If you are forced to back, and you see your lane is pushed, kill golems on the way (But ask your jungler first) as this will grant bonus gold and Exp, and won't push the lane.

Pros:
Silence
Great creep farming
Executing Ult (R)
Very tanky
Great AOE damage
Good lane sustain
High mobility/Speed
Can absorb a lot of damage

Cons:
Can be heavily countered by ranged champions
There is a chance that you might not be legendary by 10 mins.

Harass with Q early game, then move onto Q then E when you get tanker, then when you think you can kill them, Q, W if you take some damage or if they fight back, then E, Ignite, Continue to E, then chase down and R when they get below 1/4 hp (This depends on how much HP they have, as well as Magic Resist but usually 1/4 will kill them).

So:
Q - W - E - Ignite - R.

Basically just play well, Garen is a good snowballer, so try to get kills where possible, but don't overextend and remember to play defensive.
"A true warrior does not fight to win, he fights to avoid loss at all costs"
Don't make the first move, just harass with Q, sometimes E, dont over extend though, and always wait for the enemy to make a mistake, (eg, fail flash, attacked by minions, miss a long CD skill shot etc...) and if you get fed, remember you are not invincible!
Plus please be kind, vote up if you like it, thank you.