Amazing AOE AD "Q" skill , Pull enemy to his chest with "E" and multiple executions with "R" :D
first question is ;
*What makes a character as a good jungle?
-Stun,slow,change position etc.
-AoE damage (for bot lane ganks),jungle clearing
-Speed, catch running enemy
-not die if your teammate was blind bronzie and you find yourself alone with laner :D
-Finishing ability (cause some laners dont know :D)

your "Q" is enough for clear the jungle and your passive hemorrhage and stacks CR for W
you are the most amazing duelist after lvl-6 , so you can hunt enemy jungler
Sooo GANKS you are darius,when somebody saw you running towards him they will run away they afraid beacuse of your "Q"
Lvl-3 or 4 you must go mid or top lane "if not pushed near the enemmy tower" first use your Q then if available W or pull with E and then W , at same time your teammate is not little scary boy he will use his combo and then if yoou want to make an assist wait or use "Q" to finish
Bot lane ganks wait "E" for enemy escape skills ,when they use may be you can pull "E" both of them and after lvl-6 you got 1-kill 1-assist

if your team mates pushed the lanes and you can't gank for score or assist you must clear the jungle fast and you need it.
if your lanes pushed by enemy and chance for score - assist gank you need this item for speed exchange.
you will be upgrade your item to "warrior" but remember you can switch this items at no cost before "warrior" upgrade. So when you feel gank time is coming change to this.
and is the most effective lane and jung clearing item and gives you high AD and HR. Middle of the enemy team hit "W" and the passive of hydra's "extraordinary lifesteal" :D
find this usefull for the shield, the shield decrease the rate of death at fight before you use your "R" , and your "R" change the destiny of the fight :D at late you will upgrade to for better shield and gives you extra damage at lower healths. You are Darius and always stack 4-5 hemorrhage then "R" , it gives you extra AD :D
I prefer if enemy ADC or Rengar&Fiora etc. high AS not feeded :D , you will choose in this situation and say my hello! to your feeders :D
my last item, why? Darius high attack damage and tank ,if you die before you use your "R" imagine this scene " 3 enemy with low health one of your team mates alive with low health you revive %30 health and "Q" then "R.R.R" R.I.P. for enemy team D:

My first advice is combo with stun skill char like renekton-panth etc. One of my friends like to play renekton and i planned gank early, i meaned very early :D . Renekton activates his stun skill first. I clear first camp and not go for red or blue but put a totem there for thives :D , I activate first Q then lvl-2 "E" . Go to lane ,renekton dives to fight like noob :D and enemy suprised when see me at 700range , then "Q" + renekton stuns+ if enemy use ignite on renekton he will retreat a little bit or go to brushes when enemy tries to escape Darius "E" says come on!!! And your teammate start to fight again, AND FİRST BLOOD !!!
Second advice for mid lane, my friend is BRAND PRO ,he will never miss his "E&Q" stun (nearly never :D) to tips for early and late; EARLY : your Q kill the minions and when you pull the enemy "open target for BRAND etc." He starts his combo with stun. We prefer Brand and darius for the passives bleeding and burning :DDD
Late tip is about teamfights: For this you must be tanky,
-Flash near to enemy team and pull them all with E when you use "Q" at same time Brand use his W and E-Q combo, most of time you pull 2-3 enemy and your Q and brands W-E finishs the minions in pulled area, İmmediately Brand "R" and you start R with squishy enemy. İf they try to hit you your Q will be available and brand QWE is again :D , Save Brand for second combo then "R" for the others may be 1 or 2 :D

My advice ... if you are playing at bronz or silver league, you must choose a character is tanky,you must choose effective at team fights , you must choose a character like HE CAN FİNİSHeS HE CAN PROTECTS HE CAN STARTS FIGHTS.... other league players choose their jungle what the team needs :D , they can choose an assassin if team needed.