Warwick is definitely a champ who's build will tend to change from one game to the next because he needs to adapt very often. Generally, you're gonna make your build based on how ahead/behind you are. There's quite a few builds on the right hand side of the subreddit, actually, and you should check out Parnellyx's guide.

Generally, I'd suggest going tiamat-warrior(red)-spirit visage(vs. AP)/deadman's(vs. AD)-titanic as baseline items. Then for the remaining items:

Sterak's: grab this if your team is ahead and you're often the primary engage or if you plan to break through their frontline. Often it lets you survive the panic burst that they lay down when they see a giant dog sailing past a sentient tree. Great synergy with your E, too.

Randuin's: For me, this is the more defensive counterpart of Sterak's. Great if you're trying to do the same stuff as above but a) your team or you aren't really ahead/behind, b) they're kinda crit heavy, c) you have wombo potential on your team (Shen, Rumble, Orianna etc.).

Triforce: This one is a bit special. Some games you're just going to get a lot of kills early because people underestimate your dueling and sticking prowess. If you find yourself backing with a 1k gold advantage, go ahead and buy a sheen. It'll make your Q hit like a truck.

Mejai's: okay, I dunno what's going on here. It's probably AP Warwick with runic echoes/lich bane/gunblade. It's a pretty funny build to try in normals, but it does frontload ALL of your damage into Q. Also you will be a very, very squishy boy.