Start a shield against aa heavy champs such as Gnar and powerful early laners like Renekton. If you're feeling ham, go blade. Both are goo in almost any situation and the decision which to buy mostly depends on preference. Trundle has quite a strong lane and good in-built sustain from his passive and w.

Iceborn Gauntlet, Sterak's Gage, Dead Man's Plate and Spirit Visage are all core items. What you go first depends on the situation.
If you're doing well against ad, go Gauntlet first.
Well against ap, go Sterak's first.
Poorly vs ad, go Dead Man's.
Poorly vs ap, Visage.

Boots depend on the situation. Do not rush complete boots unless the boots you're going for are great against the whole enemy team.
vs aa heavy ad team go Tabi.
vs cc heavy magic dmg team go Mercs.
vs lots of slows go Swiftness.
If the enemy team has a balanced team, choose on your boots depending on who's causing most trouble to your team (etc their Vayne is fed and other's weak=>Tabi)

Your last item usually should be Banshee, Randuin or Thornmail since you have plenty of damage even without BotRK. But if you're feeling cheeky and for example are doing a lot of split pushing and look to duel people, go for BotRK.