When you receive a used keg, I would advise on the following, the best type of cleaner is sodium percarbonate based so unperfumed Oxi, neat SP, PBW, ChemClean etc.
You need water above 50c, place your chosen cleaner into the keg, add in 5 litres or so of the hot water, attach the lid and shake. Take care, hot water may spill out till enough O2 is produced to seal the keg. Periodically shake the keg over the next 10 minutes. Attach your dispense tap and push the cleaner out till the keg is empty. Open the lid, add a few litres of fresh water, seal the keg, shake then dispose of water, repeat once more but this time push the clean water out with CO2 through your dispense tap. Now add 1 litre of your chosen no rinses sanitiser, seal the lid, shake and push it out with CO2 through your dispense tap. You now have a clean, sanitised keg and tap with the bonus that it is CO2 flushed.
A couple of points. The kegs are designed to be cleaned in place, it is very rare that a keg needs a strip down, as in taking the posts off. If you do take the posts off unless you have the correct tools don't try to remove the poppets or you can cause yourself a whole host of pain. If you do take the posts off make sure they go back on the same keg on the correct in/out side.
Don't try to take the posts off unless you have the correct, 22mm deep socket.
One last thing, never, ever let anything dry on the inside of the keg. If you haven't got time to clean it, simply fill it with water.
Different brewers will have different routines but the above has served me very well.
