Not Updated For Current Season
This is my first build guide, so constructive critiscism is welcome =).
To start, let's talk about the masteries. I chose a 9/21/0 build because Trundle seems to be a bit sketchy for the first few levels. His passive is only at 2% at the start and minions do not die fast enough for it to be effective, so the defensive tree helps Trundle lane longer and be a bit more aggressive if you have the opportunity. Having defensive masteries also turns you into more of a monster when you get your ult.
Skilling order: I start with rabid bite, then at level two I pick contaminate, then pillar of filth. From there I make rabid bite priority, picking agony when I can, then I max out contaminate, finishing with pillar of filth. Rabid bite is your damage. When it is level 5 it has a low cool down, and it only costs 40 mana so you can use this move often without worrying about mana. Contaminate makes you run fast, attack fast and greatly reduces crowd control. You might want to look at this move first if you think you will be tanking more, it's all personal preferance really. All in all this ability makes Trundle hard to keep down. Pillar of Filth is a fun move to use. It works like Anivia's wall except it slows nearby enemy champs as well. Being practiced with this ability ensures that runners will not get away and runners on your team can get away.
Agony turns tanks into paper. Use this ability on tanks and they will die fast, while turning you into a beast. Even at rank one this ability can turn the tide of a team fight.
Runes: I haven't speant much time playing with runes, but I go with attack runes so Trundle has some damage. Armor pen marks, crit chance seals, attack speed glyphs and cool down quints seem to be working so far.
For the first item, rush a Tiamat. Trundle has no area of affect abilities, so getting one of these will allow you to farm minions easily, especially after you get a couple of points into contaminate. Tiamate also turns rabid bite into an area of effect attack. Innervating locket works wonders with Trundle. Paired with the cool down quints your rabid bite is going to be around a 3 second cooldown and with innervating lockets passive of regenerating mana after an ability, this move only costs 20 mana, so you can spam the hell out of it. I find with contaminate your attack speed will be plenty, so depending on the opposite team I go with either ninja tabi or merc treads. I usually grab tiamat first, then boots and then go for innervating locket.
It is after this point I get shady on what to get. If you are having a hard time keeping health I would go for an HP regen item like Force of Nature, or Randuin's Omen. If you are stomping the other team, I'd get Bloodthirster. Lifesteal is always handy on a melee champ, and the damage will help your rabid bite.
Trundle is still a very new champ. I have not found many guides on him so that is what made me write this. This build is working out for me so far, but any feed back is welcome.