Warwick is a phenomenal Champion in the right hands. However, not only does he require a very specific build to be successful, but he is also a poor jungler early game (pre-level 6, specifically) and many Champions counter him.
First off, never try to Counterjungle without help. Ever. Warwick has amazing sustain and is great at deflecting Counterjunglers, but he can't survive in the enemy Jungle against most counterpicks either.
Second, don't bother ganking until level 6. The longest ranged attack you have is your Q (Hungering Strike), and it sucks up your Mana like crazy and has too long of a cooldown to be effective in a gank. Unless your lane partner has some crazy Hard CC, do not bother.
Third, always get Blue buff (Ancient Golem), whether you're Jungling or not. The buff is a match made in Heaven for Warwick.
Fourth, don't build primarily AD or AP. AD works with his Ultimate; AP works with his Q. Get a Hextech Revolver to get AD, AP, and Lifesteal, Sorcerer's Shoes or Berserker's Greaves for the extra damage or Lifesteal from his passive (respectively), Spirit Visage and possibly either Frozen Heart or Randuin's Omen for survivability, and round out the build with either a Black Cleaver or a Bloodthirster (AD) and either a Deathfire Gasp (for Anti-Tank) or a Rabadon's Deathcap (for Tons of Damage) (AP). Another option (especially if you're Solo Top) is build mostly tanky with items like Spirit Visage, Frozen Heart, Sunfire Cape, and Aegis of the Legion (upgraded to Runic Bulwark), and add damage with items like Blade of the Ruined King and either Wit's End or Frozen Mallet. In this build, you skip boots, mostly because you'll be in teamfights as an Off-Tank/Initiator and will be absorbing/dealing out damage for assists, not chasing down targets with your E.
My end-game build is usually Sorcerer's Shoes, Spirit Visage, Frozen Heart, Sunfire Cape, Blade of the Ruined King, and Frozen Mallet. High survivability, good soft CC (slows from Frozen Heart and Frozen Mallet), the Sunfire Cape is incredibly good with my Initiate gank into the heart of an enemy team, my Boots give me bonus damage on my Q, and the BotRK is a phenomenal Anti-Tank weapon, especially against people building Atmog's (Atma's Impaler and Warmog's Armor).