There is a spreadsheet at elitistjerks.com that lets you input your gear and talents and comes up with matching rotations.
Generally, Wrath on trash, possibly Moonfire. On bosses, keep Moonfire up, use Starfire in between, and Insect Swarm when you have a GCD to spare. Only use Wrath on bosses where you have to run around a lot rather than staying in place. Insect Swarm helps on those bosses too as your DoT will be ticking while you change position.
Don't bother with Insect Swarm on trash mobs though, as they will usually be dead before it gets to complete all ticks. That cooldown can be better spent using Faerie Fire, which will help the tank with threat and melee with some extra dps. Not an issue though if you got a druid tank as they will usually pull with FF anyway.