Hello, I am a Darius' player that goes by the nickname of Frosty or Frsty.
I am going to get you to play Darius, dominate your lane, and teach YOU how to carry yourself to a higher ELO.
First off, I am 11-1 with Darius (I don't play him as much as I'd like to.)

Pros:
Extremely powerful without building damage
Easy to carry with
Tear through squishies
Snowballs hard with just 1 kill
Doesn't fall off if built right
Passive armor pen
magical damage passive
Cons:
Vulnerable to ganks if pushed
Pushes quickly if fighting in minions
Relies heavily on ultimate
Small ramp up for ultimate and damage

The runes are pretty simple, but armor penetration is VERY useful against champions like Garen that are very tanky because of their skill set.
The rest is pretty basic, armor or scaling armor depending on your lane match up (Scaling armor is good against AP tops like Vladimir and Rumble who don't deal much physical damage, aside from auto attack's)
Scaling magic resistance, obviously because AP champions scale really well and the flat runes aren't as strong.
The attack damage quintessences are a MUST for your early game so that you can deal massive damage early and farm easier.

Mastery set-ups each bring their own set of pro's and con's, but if you're a newer player run 9/21 because you really won't need to know the big difference until you get to higher ELO's or levels.
Players with a bit more experience, I expect you to know what you're doing with masteries before reading this.

Pretty simple, pick your ultimate when available and max your Q ASAP.
After that, your W is priority over E.
Your W is REALLY strong with the way it works with your bleed stacks, but not as powerful a tool as your Q.

Get your E at level 2 if you're having trouble against a ranged champion.

Do NOT take anything except the following combinations:
Ghost Ignite
Flash Ignite
Ghost Exhaust
Flash Exhaust

each has their own scenario where they shine, mostly take flash ignite unless you feel you're going to need exhaust against a champion like Jax, Fiora, or Irelia.

Your job is to be annoying while your carries kill them, or simply do it yourself.
If you're the sole person that is able to tank well (Example team: Darius, Shaco, Twisted Fate, Sona, Ezreal) build pure tank and initiate with flash > E.

If, however, you're in a rare game where you're NOT the sole tank then build bruiser and deal as much damage as you can with your Q W R.

Remember to give your carries kills! Do NOT be the only person with kills on your team, because then you're going to be focused and killed or doing no damage.

Laning phase: Shine, shine, shine!
Basically, your job is to farm until 6 while effectively zoning the enemy.
do NOT Q a minion wave and push unless you're looking to go buy.
Once you are 6, you really jump in ability to kill someone.
If you're not comftorable enough to tell when you're able to jump on someone and kill them then play normals until you can.
Champions you need to shut down early, you MUST get ahead of them.
For say, Wukong, Riven, Irelia.
These champions are ones that CANNOT be allowed to keep up with in farm, but undoubtedly you're stronger early game. Try and get them to trade with you in YOUR minions, poke at them, harass them, keep them our of lane when available.
Do NOT push turret too early, that will just allow them to free farm.

Some lanes, you just need to stay within range of experience but stay outside of the enemies range.
This means, stay in a bush and let your lane get pushed and only pick up CS when available and when the enemy is MIA.

Darius can effectively roam to mid lane, so make sure you do when possible as well as stealing enemy buffs!

If you've done your job successfully you should either be fed, slightly ahead, or even with your enemy top laner.
Your job here is to PROTECT your mid lane carry (AD casters or AP carry) and your AD carry.
While protecting them, deal as much damage to the enemy carries as possible (I.E. E them to you!)

Another good option is to split push and force them to either initiate 4v5 and lose a tower, have them all come after you and lose a tower, or send someone to try and stop you.
The ONLY time you should split push is if your team is ahead.

This is where you shine the most.
You can dunk on anyone in a 1v1 scenario, push extremely well, tank through anything (generally) and ensure a victory.
Again, your job is to protect YOUR carries while killing theirs. If you can save your E to protect your carry from their tanks, you just saved a team fight in most cases.

Just auto-attack minions when they're low, do NOT Q a wave unless you're pushing so you may back or to split push. If you fear you're going to miss a minion, hit your target then W for the auto-attack reset.

If you're sitting under the turret, allow the turret to hit the minion twice then attack it.

Personal or in-depth questions?
Add me on skype, my username is Frstypwn.
I'll answer them as best as possible.