Firstly, you should ensure your radiator is switched off for safety reasons before commencing with cleaning your radiator.
It is important to remove dust from in between radiator panels, so as to not prevent the distribution of heat, and ensure they are performing to optimal capacity.
To do so, you should fashion a cleaning device from a stick or thin plank, with a towel or cloth wrapped around the end. This will allow you to reach into narrow crevasses and eliminate dirt and dust that could cause potential blockages.
With regards to the actual radiator surface, you can use a sponge and some soapy water to tackle any stains or markings.
Get more radiator cleaning advice in our full guide, How to Clean a Radiator.