Iron soup kettle with rust
I'm assuming that you're referring to a cast-iron kettle. One of the common strategies for removing rust from cast-iron is to rub the oxidized areas with steel wool. You'll want to remove as much of the rust as possible that way, and then you can consider using a wire or plastic brush, soap and water.

Subsequently, once you're confident you've removed the rust, brushing with food-grade flaxseed oil (or shortening, in a pinch) and heating the pot for a period of time, then letting it cool so that the coating is mostly polymerized, will help reduce the likelihood of future rust development.

There is a guide to restoring cast-iron covering roughly the same set of steps.

The reason for preferring flaxseed oil is its polymerization properties as noted above, but you may want to see a more detailed explanation.