Acid based sterilisers (either a generic no-rinse solution or Star San) are recommended over chlorine based ones (VWP) as the chlorine can cause pitting in the stainless steel. Plus there's the added bonus of not having to rinse.
Always release pressure from your keg before opening the lid. Pull the pressure release valve or, if your keg doesn't have one, push down on the centre of the 'In' post. Once the pressure's released pull up the lid handle and hit the lid with the heel of your hand to open cleanly, then twist the lid to remove. Rinse the inside thoroughly with warm water to get rid of any remaining beer and residue. You may want to do this a couple of times and get your arm in there if you can see visible beer remains and/or 'tide' marks. As always with cleaning homebrew equipment, the sooner you clean the keg the easier the job is going to be!
Following the steriliser dilution ratio as instructed on the product packaging. Add around 5 litres of sterilising solution to the keg, replace the lid and give the keg a good shake to make sure steriliser comes into contact with all internal surfaces of the keg. Pressurise the barrel up to 10 psi then open the tap to flush steriliser through the dip tube and tap. Now the keg is clean and can be stored. It is best stored under pressure... or, better still, fill it back up with a fresh brew.
It is worth fully breaking down your keg every 6 months or 3-4 uses to give the parts a more thorough clean and check all the sealing washers and 'O' rings are in good condition.