Hello and thank you for taking the time to view my guide. I should tell you that this is my first guide; but I hope it helps you. I love to play as a lot of different champions but Darius is usually the one I always come back to. He just seems to have the perfect combination of skills and balanced stats.

This is helpful throughout the game. With the mastery it gives you 5 extra damage which is useful early game. Also it gives you that extra damage that can get you the kill.

This is also a good multipurpose spell. You can use it to quickly escape death by turret or champion after getting a quick kill. Or it can be used to jump past a turret to snag prey that has gotten away with only a little health left. Although that is often a very risky move.

Fairly basic beginning. Boots to start with, add health potions if you need them. Also some people like to use Doran's Blade early game as well. I personally don't but that's up to you, the end result is the same.

After that we get Ionian Boots of Lucidity. I like these because they allow you to use Darius's skills faster. But if you need more defense you can use Mercury's Treads. Again these first items are flexible.

Then we go into mid game where we try to deal a lot of damage quickly, so we get [zeal]. After that we need some defenses so we get Vampiric Scepter as well as Rejuvenation Bead which will lead to Tiamat. This is personally one of my favorite items because it gives you damage, health regen, and mana regen all in one with an extra kicker of splash damage to clear crowds of minions quickly.

Once we get later into the game things get heavy real quick. First you should upgrade into Phantom Dancer for a little extra boost. Then you explode into AD with two Bloodthirster or if you don't need any more defensive life steal you can exchange one of those for Infinity Edge. Finally if you have time you can get one more Phantom Dancer and you'll be unstoppable.

http://www.mobafire.com/league-of-legends/mastery-tree-planner#&tree1=1-3-0-0-4-4-1-4-1-0-3-1-3-0-3-0-1&tree2=0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0&tree3=1-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0&v=2

These runes all work together to let Darius get the most out of his basic attacks.
You will gain attack speed and critical hit chance so that you can deal as much damage as possible as fast as possible. You need to kill them first before they can do any decent damage against. Your offensive power is your defense.
http://3.bp.blogspot.com/-J2o-K4sLQCo/T7f-f0NLe_I/AAAAAAAAAFw/QUsrN27Lr6s/s1600/darius_splash_12.jpg

The same idea goes into the masteries. The faster you can kill them the less damage they can deal to you. So, of course, I put everything into the offensive tree(except for one in utulities to reduce flash's cooldown).
http://www.mobafire.com/league-of-legends/mastery-tree-planner#&tree1=1-3-0-0-4-4-1-4-1-0-3-1-3-0-3-0-1&tree2=0-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0&tree3=1-0-0-0-0-0-0-0-0-0-0-0-0-0-0-0&v=2

You should of course get all of your skills first, because they work as a combo; but I focus on Apprehend first. Although you should always level Noxian Guillotine ASAP.

This skill is one of your best assets. It allows you to take control of the situation. You can pull champions away from their turret, or pull them back to you in a chase. Also the armor penetration is wonderful early game when the other champions haven't built up their defenses yet.

This skill comes in a very close second to apprehend. It will quickly ramp up in damage with your AD as well as slow your enemies as you begin to chase.

Decimate is a big damage dealer with added range if your prey is pulling away from you.

The basic combo here is to run up to your prey and begin with basic attacks and a Crippling Strike. Then once they try to run you pull them back with Apprehend and slow them with another Crippling Strike. After they start running again you use your Decimate at blade length for maximum damage. Finally you finish them off with your Noxian Guillotine and if necessary you then Flash back to safety.

The general idea of this guide is to give Darius a balance of attack speed and damage which will both play off of each other for maximum destruction. However this destruction is geared towards the enemy champions not the turrets. You are pretty much an offensive support. You should not solo a lane for this reason, you need a ranged AD Carry to lane with you. Some good partners would be Varus, Ashe, or Miss Fortune. They will help thin the minions and will be doing most of the damage to the turret. As the game progresses you should be pushing very quickly down the lane keeping the other players off of your ranged carry. Because of this though, the other team may heavily target you in which case you may need a little more help from a ranged mage with good AOEs such as Karthus.
You will also needs to choose your battles wisely. Always focus one champion at a time. If there are multiple enemy champions in your lane be careful because you might be able kill one of them but because this build leaves Darius a little squishy any other champions will surely kill you.