@Boof - Generally for trundle you want to spec defensive masteries (0-21-9, or a mix of defensive and utility [so you can get the movespeed]) and run Armor Pen reds (can mix in AD if you want), Dodge or Flat Armor yellows, Flat MR blues for duo lane, scaling MR/level for solo or jungle, and Quints can be anything from Armor Pen, to Movespeed to Flat AD.
For jungling I run either 1/14/15 or 1/19/10 depending on if the team needs more dps or tankiness, and I run 10 armor pen, 9.6 AD, flat yellow, scaling mr. Open cloth and 5 pots and start at blue (he's very mana hungry early. You can jungle w/o it but it will be much slower).
If you are laning your first major item should be a phage, and then you can either build into Tri-Force or Mallet depending on, again, if your team needs DPS or Tankiness, and then proceed to build Atma's/Fon, etc.
General tips and tricks - Use your Q right after an auto-attack to reset it and do damage twice as frequently, your Q can also be used immediately after Contaminate starts casting to negate the animation. Don't place contaminate until your team is committing to a fight, b/c you need it for survivability, mobility, and damage. Use pillar to either split their team up or protect your carry.