First Time Darius Dunking Apprentice Level Noob.

BASIC:

Extend duels for as long as possible. The longer a fight occurs, the better your chance of winning. Darius is weak in burst trades since he can't abuse the power of his passive (hemorrhage).

EARLY GAME:

Early game, always try to use your Q to either harass or initiate. Auto attack an enemy and immediately follow up with crippling strike since it resets your auto attack timer.

Early game, use apprehend when being ganked on both laners to set yourself up for a double decimate sustain. This can either allow you precious time to run or change the fight in your favor.

Understand just how powerful you are as a juggernaut. Riot blessed your champion with some of the absolute best base stats in the game. You have among the best: base HP, base damage, base HP regen

#1 RANGED MATCHUPS

Typically, these are your unfavorable matchups. Start with Q. With some ranged champions like Kayle (I understand she's technically melee) or teemo, their auto attack range is barely longer than your decimate. Every once in a while you may find an opportunity to take a step in when they go to harass you and get a decimate off. Every time you trade a Q for an AA you win the trade (Especially since no ranged champion in the game is as durable as Darius).

Get apprehend as your second ability. This is your punish ability. Most ranged matchups (including melee champions who rely on ranged harass like gangplank or Pantheon or Kayle) involve champions who have to get within apprehend range to land auto attacks or harassment spells. If you can punish them with an apprehend, they will usually lose the trade heavily or even die to a darius all-in.

#2 MELEE MATCHUPS

These are usually Darius's easy matchups. Darius can potentially bully: Garen, Mundo, Maokai, Sion, Aatrox, Trundle (caution, this is a skill matchup), Jax (skill matchup in Darius's favor), Galio, Yorick, Nasus, Tryndamere, Mordekaiser, Illaoi (caution, so long as she doesn't pull your spirit and ult you can win, BEWARE ILLAOI ULTIMATE), Irelia (Caution, now a skill matchup that can easily turn in Irelia's favor with new triforce and slightly empowered Q), and Yasuo (Caution, do not let him snowball if he does happen to get first blood)

Start Q in most of these matchups. Some matchups like Nasus or tryndamere you may want to start W since it quickly applies two stacks when used correctly and doesn't push the wave really hard like decimate does.

Use everything you can before using apprehend. YOU WANT TO USE APPREHEND AS A REENGAGE TOOL, NOT AN ENGAGE TOOL.

MID GAME:

Your ultimate level 1 is relatively weak. It can be very powerful in duels and Darius vs 2 champs ganks, but otherwise, in team fights, it can be hard to try to make guillotine useful since it usually maxes out at around 360 true damage if you can get 5 stacks.

Your ultimate level 2 is a powerful tool. Maxing out at around 675-780 true damage (depending on build and dragons), this ability can effectively one shot squishies if you have noxian might and decimate them before using Noxian guillotine. Level 2 Noxian Guillotine is when Darius's potential teamfighting can become scary if he gets a reset. In teamfights, you typically want to 5 stack a tank before moving past him to try and reach the backside of the enemy team with a decimate. When moving as a team, don't be afraid to use apprehend as an engage tool since a caught out enemy is usually a dead one, even tanks (unless they're fed). Your optimum combo is as follows ---> Q > AA > W > AA > Q (5 stacks). Your typical combo is as follows --> E > AA > W > Q (3 stacks)

LATE GAME:

Rejoice! The Juggernaut update gave Darius a chance at having a meaningful late game (Old Darius was worthless at this stage). Your ult typically maxes out at around 1000-1150 true damage, and becomes an incredibly powerful chunk/execute tool whether it's used on a tank or a squishy (it is the absolute strongest instant true damage ability in the entire game, feel the power at your fingertips!).

Teamfights are the same as mid game, you want to try and 5 stack a tank before moving on. Beware carries who become powerful enough to severely chunk your ass in mere seconds.

GENERAL TIPS:

Play greedy, it's no longer a bad thing for Darius to take kills in teamfights with his ult since he now has the noxian might passive.

Early game your most powerful skill is your passive, so make sure you put it to good use.