It's always best to start by removing any loose or flaky rust by hand with a wire brush. For stubborn areas, try a cupped wire brush in a drill or an angle grinder. Once the bulk of the rusted material has been removed, you can coat the affected areas with a rust converter which chemically reacts with the rust and neutralises it. If you're planning on coating the area, you can now prime the surfaces before painting them.
Sometimes just a wire brush is enough before repainting. Next step I'd try is just something like vinegar on the brush or a scourer, before moving to a rust remover like Chemtech 1L Rust Off Remover. - MartyH