Warwick is a very strong solo laner with great sustain. He is very tanky while dealing high magic DPS. His ultimate is amazing to focus the AD-Carry and forcing Baron Nashor is no risk, because Warwick won't get any damage. All in all a well played WW can be game changing.

Not much to say about the runes: Armor and Magic Resistence per lvl is a must have on every Bruiser or Tank. On WW i prefer AS quints and marks, because you will deal your main damage with onhit-effects.

For Masteries I get 9/21/0. If your enemies won't have much CC you can also put the "Tenacious"-Points to "Legendary Armor"

Start Boots+Pots and get a Wit's End first. If u play vs AD, you should get a Glacial Shroud now.
If you play vs AP take Mercury's Threads before. Get Phage for some HP and the slow. Yor last Core item is bilgewater cutlass, which you can upgrade to Blade of the Ruined King, if they get high HP.
If they have high physical damage take Frozen Heart first otherwise Spirit Visage. Then upgrade Phage to Frozen Mallet and finish ur build with Tier 3 Boots, if u didn't get them earlier. I prefer Captain Enchantment, but the other ones are also fine. This is very situational.

Max Q first, because u won't need any items to deal dmg with it and it will be usefull the whole time. Then max W to get the AS for your Onhit-Items. Max E last and be sure you always skill ult when possible.

Flash is a must have and Ignite is also very important. It's possible to get Teleport instead of Ignite.

In Teamfight you can initiate very good. Just use your Ultimate on their AD-Carry in the right moment and you can destroy them easily.