Hello, and welcome to my first guide. I'm going to run through the way that I play Darius, and show you the item/mastery/rune choices that I've made whenever I've played him. I am fully aware that there are other ways to play Darius, and I've played several of them, so I will be justifying my choices for each item/mastery/rune in the relevant section.

Darius is a melee ranged fighter that uses a mix of physical, magic and true damage. He is fairly versatile, being able to fill the role of melee carry, bruiser or sometimes offtank. However, building him as a melee carry or offtank, to my mind, wastes his enormous potential for destroying people as the melee roles is far too squishy and will be instantly blown up in a teamfight, while the offtank role generally doesnt build enough damage to get to the backline and kill the carries on the enemy team.

There is, however, a slight problem with Darius that is unrelated to either his skillset, the skill of the person playing him or the enemy team composition. Since a fed Darius is such an intimidating presence in a game, many teams will go out of their way to shut Darius down hard even if this means they neglect focus onto other lanes. Put simply, people hate Darius, and will go to extreme measures to kill him, even if this means a 4 on 1 or even 5 on 1 gank at times. Be prepared for this.

The Pro's and Con's of Darius:

Pros;

If built correctly, can deal massive damage to multiple or single targets
One of the hardest snowballing champions in the league, if he gets ahead he's stupidly hard to stop
Is reasonably hard to shut down, very few "hard" counters
Quite easy to gank for due to Apprehend.

Cons;

If he gets shut down, then he can be a real hinderance to your team
Skillset forces you to take flash as he has no other escapes otherwise
While he has a low entrance difficulty, it's hard to master the range of both his Decimate and his Apprehend

On Darius, I set up my runes to provide the most benefit for the early/mid game, since that's where Darius either needs to snowball, or survive numerous gank attempts.

I take greater uintessence of desolationto improve my early damage output. Pre lvl6, darius deals nearly all of his damage as physical damage (since he does not yet have his ultimate and his passive needs a lot bonus attack damage to scale up well) and given that a lot of champions do not have large ammounts of armour early on (unless they invest runes/gold into buying extra) Darius can deal good damage early on with these quintessences

For marks, I take flat attack damage. Essentially, this means that I dont need to buy a dorans blade early to help with last hitting.

For glyphs, I take flat magic resist to mitage early magic damage, since quite a few top lane champions deal some magic damage.

For seals, I take flat armour glyphs, to mitigate early physical damage. If I'm facing an AP champion in the top lane (Vladimir, Rumble, Kennan), I might choose to swap them out for scaling magic resist seals to ensure a safeish lane

When I play Darius, I use a 9/21/0 mastery build, taking the bonus resistence, health, health regen, cooldown reduction and damage reduction masteries in defence and the increased % armour penetration in offence. This build gives me the durability needed to trade with opponents in lane, gives me increased sustain while in lane and also gives me some much needed offensive capability.

Why not 21/9/0 or 21/0/9?
While it is true that Darius can be built with a full offence built that incorporates some defence (a typical AD carry build) or a full offence build that includes extended neutral buff duration, from my experience it really isnt worth it. While having the 21 points in offence does increase your damage somewhat, when I swapped out the my 9/21/0 masteries (actually at the time the setup I used for Irelia) I didnt actually notice any difference in the damage I was putting out, and the extra surviability was just so noticeable.
I did try out the 21/0/9 build (taking the extra mana and mana regen masteries as well as extended neutral buff duration) but on Darius I really dont seem to run out of mana that much unless I constantly spam my skills, which you really dont need to do.

Starting items:

On darius, I start with boots of speed and 3 health potions. For me, this gives me the best sustain early on in lane (so long as I dont engage in too many early engagements before my power starts to ramp up) as opposed to a doran's blade or a doran's shield. While the blade does give me some damage, health and lifesteal, the fact it doesnt build into anything useful later on makes it a less attractive option for starting off.
Truth is, I havent actually tested the Doran's shield, but I havent had a problem with sustain while using boots and three health potions, so really it's down to personal prefence.

Early game purchases:

On my first return to base, I generally pick up a philosophers stone, a vision ward and a few health potions if I have any money left over. After this, I generally get a heart of gold or a wriggles lanturn. I get both of them early on, but the order I get them in depends on how well I'm doing in lane. If I have an advantage over my opponent (either through a sucessful gank or simply outlaning them) I get a heart of gold, but if I'm struggling in the lane (either from getting ganked or losing out to my lane opponent) I'll go straight for the lanturn.

Mid game purchases:

By the time I reach the beginning of late game, I should have the money for a phage (and a hexdrinker if I've managed to pick up some kills early on). These two items should give you the damage you need while you build up to your warmogs armour. At the end of mid game (just before the major teamfights start) you should be working on your atmas impaler (or, if you're doing very well, you should have finished it)

Late game purchases:

In terms of items, this part of the game is the most variable. There are three items you should be aiming for. A frozen mallet (upgraded from the phage you bought earlier), a maw of malmortius (upgraded from your hexdrinker) and a bloodthirster (bought by selling your wriggles lanturn). The order that you get these items in is entirely dependant on what you need at this point. If you need extra health and sticking power, get a frozen mallet first. If you need more magic resist ( because the enemy AP carry is dominating) then get a maw of malmortius. If you need more raw damage, or you dont need the extra survivability grated by the other two items, get a bloodthirster.

Once you have your full build, you should pick up all three elixers (fortitude, brilliance and agility) for extra health, damage, cooldown reduction and crit chance

Frozen Mallet

While I generally buy a frozen mallet fairly late on, there are some arguements for getting it early. Really, an early frozen mallet is best when you're unsure in your use of Apprehend or if you're facing an opponent whom you can kill but has a good escape mechanism (eg Kennan). However, once you become more confident with your use of Apprehend, you should be able to rely less on the frozen mallet to keep your opponent from escaping.

For Darius, taking Decimate at lvl 1 is my personal preference, but I cant think of any reason you'd take Crippling Strike or Apprehend first unless you're going for an early invade and you want as much cc as possible. That said, if you have a character on your team that has a reliable cc at lvl 1 (morgana, blitzcrank, alister, taric) then you can take Decimate at lvl 1 for an invade.

I max out Decimate by lvl 9, since in lane it gives the best harrasment onto enemy champions (with correct useage of the outer edge mechanic) and gives great farming potential at higher ranks with some attack damage. You should practise hitting champions with the edge of the blade for maximum effectiveness, but dont be afraid of hitting it while the champion is next to you in a straight fight.
Never pull someone in with Apprehend and then use Decimate. Always use Decimate and then use Apprehend to pull them in.

I max Crippling Strike second, by lvl 13, since it is a good source of damage and cc but is not required early game since your auto attacks wont make up that much of your damage early on. You generally want to delay the use of crippling strike in 1 on 1 engagements, since using it with 4 or 5 stacks of Hemorrhage applied to them will allow you to spam the skill with a much lower cooldown. The exceptions to this is when you are fighting a champion that relies on attack speed (Irelia, Fiora) or when you know they are going to run straight away instead of stand and fight

I max Apprehend last, but I take a point in it early so I can take advantage of the utility. The passive bonus is good, but not amazingly required until late game, where champions start to get large ammounts of armour.
Good use of Apprehend is paramount to playing Darius well. It doesnt matter that you can use Decimate perfectly every time if you screw up your grabs because you'll just never catch anyone. The main mistake made with Apprehend is that people use it to initaite fights. While it is effective at doing this, this should only be done in a few situations (pulling someone over a wall in the jungle, pulling someone in to bait them into a counter gank). Other than those situations, the most effective use of Apprehend is to pull enemy champions back in once they have disengaged. This is especially effective against champions such as Irelia who have no escapes outside of flash, as you can simply pull them back in once they have flashed.

Whenever my ultimate is available, I always take a point in it. While I am aware there are champions who sometimes delay getting their ultimate in order to max one of their skills faster, Darius is not one of those champions.
Many people will play Darius as a melee version of Karthus and killsteal with the ultimate. While you can do that, just because the ultimate refreshes upon getting a killing blow does not mean that that is all you use it for. It can be used in a 1 on 1 fight where you are losing in order to turn the fight around, it can be used in a gank/countergank to softed up a tanky target in order to let a harder carry take the kill and it can be used in a teamfight to do maximum damage to one of their carries who is doing particularly well.
That said, taking as many kills as you can is not an entirely terrible strategy as long as you maintain the snowball and dont starve out the other carries on your team. If he gets far enough ahead, Darius can carry a game hard, but dont rely on that happening since League of Legends is a team game and you cant win 1 v 5.

For summoner spells, I generally take Exhaust/Flash or Teleport/Flash when in top lane, while sometimes taking Ignite/Flash

Exhaust/Flash combo.

I take this combo when I'm facing an opponent that relies heavily on trading with me or auto attacks as their main form of damage (Irelia, Tryndamere, Fiora) since this means that I can overpower them in a 1 on 1 fight and force them to try and run or stop them from escaping when my jungler is ganking if we dont bring enough cc (if the jungler is someone like Shyvana or Gankplank)

Teleport/Flash combo.

I take this combo when I know I'll overpower my opponent without needed ignite or exhaust and so will be able to go to other lanes with teleport and gank without warning, when I'm facing someone else running teleport so they wont be able to help out in other lanes without me coming in to stop them, when I'm facing Shen (although Darius can stop him altogether by using Apprehend to break the channel. Even if you have to burn flash to do it, it's worth it as he wont be able to invervene in a fight in another lane and possibly turn the fight in their favour) or when I'm facing a very hard pushing champion with good harass (Kayle or Yorick) and I dont want to be out of lane for a long time when I purchase items.

Basically, the above build is how I play Darius. I stress this is not set in stone, and the runes, masteries and items choices are entirely up to you. I do provide comments and advice on how to use his skills properly, but again you might disagree with me and use them differently.
I would definitely recommend giving Darius a go. He's a very fun champion to play that doesnt require that much skill with him to play (although he's slightly tricky to master) and of course, he has the potential to be a complete monster at all stages of the game.

More sections will be added soon (as soon as I finish them), mainly a list of good and bad lane opponents for Darius and a teamfighting guide. Hope you have fun.