Controlling a heater

Hey, guys. I'd like to be able to control an immersion heater's power, without modifying the heater itself. I live in the US, so I believe wall power is supplied at 110VAC. What hardware is recommended? Shields are much preferred, if possible.

You want to look for a SSR - Solid State Relay - these connect direct to the arduino and act as a switch to the heater.
As you are playing with mains be sure you know what you are doing or get someone qualified to help.

Assuming what you are trying to control plugs into your standard AC outlet, Another option would be the Power Switch Tail. It’s more expensive than a relay, but there is the added safety in that you don’t have to bother with any high voltage wiring.

How many watts does the heater have?