Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Okay this is my first build and it works really well for me.
I start out with Regrowth Pendant and a health potion. The health Regen on the item along with your passive will make you that much harder to kill early game. Then when i get enough money, which you should use your q to get minion kills just don't overuse it (which can easily be done), I get the mercury treads, which again works well with his w ability, and makes him hard to kill (what I like to do is push turrets really far and when they try to gank me flash away use my contaminate and throw down my pillar of filth so they can't get me). After that build into Warmog's armor as soon as possible, like the minute you have enough money for it buy it. It is essential to build into your warmogs for the extra survivability. Next build the Frozen Mallet, slows enemy and give you extra health, which at level 16 if you have correctly maxed out warmogs and build the frozen mallet you should have 4k health. Then go with the Wriggle's Lantern, which I really like because it adds a third regen on Trundle. Then the last two items not only give you attack damage, but they do well and give you that extra survivability late game.
The way I play trundle is pushing lanes. Lane control wins games, in my opinion. So push lanes, and use your contaminate on the turrets, but make sure to steal attack damage from a minion every time you get a chance. In team fights, wait to use your ult until your team has killed all the other carries, then, if you haven't used your ignite, use both on the tank. It will die fast. If you are soloing someone, wait til you get half health, then pop your ult and ignite on them, especially tanks, they think they will have you, but you should be able to kill him.
That's pretty much it, pretty fun character to play in my opinion. Comments are always fun, along with suggestions, I always like to hear how other people succeed with my favorite champions.