If you have a rusty cast iron skillet or dutch oven the easiest way to remove the rust is by running it through the self-cleaning cycle found in most modern ovens; by the end of the cleaning cycle any rust is turned to ash. This effectively "resets" your cookware and you will have to season it before you cook with it.
http://lifehacker.com/cast-iron/Learn to Season Your Cast-Iron Skillet...
http://lifehacker.com/5288584/learn-to-season-your-cast+iron-skillet