As Darius, there are two things to focus on whilst in early game.
The first thing is to retrive as much gold as possible from the minions. Thanks to the more defensive build you will be able to take the damage from the minions and will therefor be able to avoid turret from stealing your gold. Of course this doesn't mean that you should tank the minions at all times and if you have your own minions by your side, let them take the damage instead.

The second thing to keep in your head when playing as Darius is to keep your enemies unable to lasthit the minions. This will give you a great advantage at lane as you level up and thanks to an early point in decimate you won't have any troubles making them stay away.

I've chosen to start with boots of speed and three Hp potions to provide myself with speed both for chasing and occasionally run away from the enemy. Thanks to the boots I will have no problem beating my opponents up and then quickly return to safety making them unable to hit back, but if you should get hit, don't worry. You will have three Hp potions which will make you able to recover a total of 450Hp in no time. This makes you able to stay at lane for a longer time and through that, out level your opponents.

After the first purchase I would be back at base when I have reached 1275 gold which makes me able to purchase both a doran's blade and a philosophers stone, giving me higher Hp and Sp regeneration, increased damage amongst with some Hp and lifesteal. This will give you some kind of advantage at lane and the philosophers stone will definately make you able to hold your lane longer than anyone else.

When you have purchased your pilosophers stone and a doran's blade, you should head back to your lane again. Stay there and farm the minions whilst pushing away your opponent until you either have enough money to buy the second grade boots which are either ninja tabi when facing a lot of ad or mercury treads when you're facing ap and cc. The boots should anyways be bought not too far into the game otherwise you risk being outrunned which will lead to a high number of deaths and too few kills.

Build up the end items, as far as I've been playing I usually have about 8-14 kills at level 13 which has provided me with plenty of gold together with the minions and if anyone gives you the opportunity to kill dragon, take it and make another 215 gold out of it. He makes a really good Damage dealer with the true damage of his ulti and makes a strong semitank too, but remember, he should not engage first, let someone else take the damage to let you make the damage.

The pros and cons of this build are:

Pros

High HP, able to take damage.
High def/m.res able to tank some without being too worried about getting killed.
About 16hp/sec in reg
Decent movement speed.
Great teamplayer.
Easier to take down other players one versus one if build the "Mercury Treads".
Overall a pretty balanced build for those who are yet to specialize in Darius.

Cons

Low damage in comparison with other Darius (full dps) builds.
No lifesteal.
May or may not be dependant of team (this is based on how you're playing)
May be hard to play if countered with CC or such.