I cant believe that no one yet has stated the proper build yet.
You start with a Catalyst and a Health Potion. You basically leave the Catalyst alone until you can't upgrade any other slot. The reason is, you level so quickly early, and throughout the whole game, that you are constantly getting the proc. You greatly outlast people with it, especially before the first time back.
After the Catalyst, you get your Boots up, either Ionian Boots of Lucidity or Merc Treads, depending on need for MR and CC Reduction.
Your next item is Spirit Visage. The whole point of the build is to create massive sustain so that you can push your opponent out of the lane, forcing a second or even a third to come and help, which greatly lessens the pressure on Top for your teammates.
After you have Catalyst/Boots/Spirit Visage up, start building Trinity Force, starting with Sheen, and then Phage.
After you have that up, get a Warmog's Armor. With Catalyst/Spirit Visage/Warmog's/Trinity Force, you basically never have to go back.
When you need to, turn the Catalyst into Odin's Veil. Finally, build Atma's to get you some armor, and to turn all that HP into AD. Final build in list form is:
Ionian Boots of Lucidity/Mercury Treads