It really depends on the enemy team set-up and their builds.
First ask yourself what boots you need: Ninja Tabi or Mercury Treads. Don't be afraid to switch boots later on if you realize you made a mistake here.
From here on, your build should depend on the primary source of damage from the enemy team.
Mainly physical damage: Randuin's Omen, Sunfire Cape, Thornmail, Guardian Angel (Don't forget to use Randuin's Active Ability; it's a nice one!)
Mainly magic damage: Banshee's Veil, Force of Nature, Guardian Angel
If they have mixed damage, just take some from both.
You could also put an Aegis of the Legion in somewhere, but that item is preferred on a different character.
You are also expected to buy an Oracle if the enemy team has a stealth character.
You can't really tell a tank what to build; it's all situational. Banshee's Veil is a solid choice against almost any team composition. Check which enemy has most kills (Both human and minion kills) to decide whether you should focus M.Resist or Armor.
I wouldn't recommend any AP, AD, or similar stats: Go full tank.
Note on Guardian Angel: It's not supposed to be an 'Oh, I died! Luckily I have a Guardian Angel'-item. You die on purpose, let the enemy waste all their skills on you while the rest of your team picks them off, and you happily survive because of your GA.