Here's how I build Darius, and with this Darius can become very powerful by end game. This build may not work for everyone but hopefully this will help you. I have never played a tanky character, I usually go for DPS or ranged but this champion is really great for sustain. He can get a little tanky at end-game. People complain about his grace period and bleed out. But, there's always a way to counter it.
We don't talk about that here, yet though. ;)

Whenever you're going to build your second Bloodthirster be sure to sell the Wriggles Lantern.
Core Items: Trinity Force Infinity Edge Bloodthirster

Trinity Force from what I was told, and have personally seen, is good for Darius. The ability power may be useless for him but everything else is actually great for him, the resist, attack damage, etc.
Infinity Edge for the extra damage of course.
Bloodthirster for the lifesteal which is great for Darius, well more like any champion that can use it. But, I like to get 2 at end game.
Black Cleaver It reduces the armor so it makes people who are building armor a bit squishier but if you have to you can also build Last Whisper replacing the second Bloodthirster.
Wriggle's Lantern It's good for beginning because you get some lifesteal from it and a free ward to put at Baron or Dragon if you need to. I usually sell this at late game for the second Bloodthirster.

If you have to get Last Whisper, the build should look somewhat the same at the beginning but after Bloodthirster go ahead and build Last Whisper First then Black Cleaver after that. Then you can sell Wriggle's Lantern to get Infinity Edge.

If you are in need of some magic resist, I would suggest replacing Black Cleaver with Maw of Malmortius so you aren't losing the attack damage. But if you absolutely need to, you can get Banshee's Veil instead.

Here are some of the summoner spells I suggest:
Ignite It helps with your bleed, in case their health doesn't go down as far as you want it to
Exhaust To slow your enemy, his Crippling Strike doesn't last as long but still useful
Flash To catch up to your enemy that is getting away
Ghost To get through the minions of course and to catch up
Teleport I suggest this because sometimes the enemy might try to get to your Turret once they realize your gone and its "safe" so this can be useful

NOW! You can counter him.
I know a lot of people wonder how to counter, but its really simple. To counter him is to have someone who can snare/stun. OR! Someone who can slow like Ashe. If he can't get the kills he needs he won't be able to build up his power. So, snare/stun (or even slow) - Gank him with at least 3 people (or the whole team if you prefer). He can't really take on that many people unless depending on the build. Also having Jarvan IV and Wukong in the same lane against Darius will give him a hard time as well. Making it harder for him to get the kills he needs to get bigger.

Point is, if hes big, FOCUS. If its early game, dont let him feed! Any snare/stun will do.

If you have any suggestions or if I should make any changes that you think seem fit please feel free to share. Also, it's good to have a champion with you that can harass or be the bait for you to get the kill. Someone like Katarina or someone who can stun for just a few seconds like Riven. Varus and Jarvan IV are also good match ups with Darius.

Also, here's a picture of my Match History: