I'm 4/0 with Trundle in ranked, sitting at 1512 until the season is over. I play him often, as he's now my favorite champion.
There are a lot of things you should know about him before playing him. Firstly, he's not OP. He's a secretly "strong" champ but he has some very clear weaknesses.
1. He's not a tank.
2. He's a very good jungler.
3. What makes him a good jungler is that he is a great duelist.. This means he favors an aggressive jungle style. This works best with coordinated teams. Noob teams will leave you hanging out to dry if you get caught in someone elses jungle. Good teams will not.
4. He does not scale well into late game.
See #4, but for more details:
The Animation on his Q and his W often cause him problems. They cannot be cast while moving so they can force you to lose ground while casing someone.
Furthermore, his Q steals a "flat" amount of AD. This is one of the major reasons he does not scale well into late game. Riot should fix the animations for Q and W and make his Q steal a % of enemy AD. This would make him considerably better, late game.
His R is very nice, but I would like to see it be a litttttle more front loaded and possibly last slightly longer.
His W should have some benefit for the team. Currently, his W is only useful for himself.. Which reduces his effectiveness in team fights. I think your team gaining 50% of the bonus that Trundle gets from contaminated ground would make it (and him) much more useful in team fights.
All of that said... here's my build:
AD/ARpen mixed Reds/Quints.
Flat Armor Yellows
Flash/Smite for summoners.
Start Boots 3 pots
Do Blue, steal enemy red, gank bot/top depending on whether you are blue/purple team, and who you are matched against.
If you are matched against someone you absolutely should not counter jungle (Skarner and Udyr come to mind), I would go Vampiric Scepter, farm my own jungle, and gank accordingly.
Boots + 3 pots, Wriggles, HoG, Phage.
Somewhere in there if I got first blood, a lot of assists, or just a kill or two, I will definitely buy an Oracles and do a lot of ward clearing. Only do this once you are comfortable with Trundle enough to not die (often).
He has a great disengage with his E, but he has no real "initiator". Keep that in mind. He is difficult to chase for most champions, but there are a few who can ruin your day no matter how slick you are.
Merc Treads, Wriggles, Trinity Force, Randuins Omen, Banshees Veil
Last item's.... it's a toss up. FoN, Last Whisper, Maw of Malmortius, etc. Any MR and/or Damage item is good. At some point you could sell your wriggles and possibly pick up a Blood Thirster if you want... but most games do not last that long.