If you have a combi boiler, you will only have to turn the cold main off and the hot will also be off . If you have a convensional boiler your main stop tap will turn of the cold and you should be able to turn off the hot via a gate valve in the airing cupboard. If you can't find the gate valve, just let the hot tap run until the header tank in the loft is empty, It will probably take about ten minutes to drain. When you turn the cold main back on the header tank will fill automatically.
You can remove a radiator by tuning of both valves either side and undoing the large chrome nuts on the rad. Remember that there will be water in the actual radiator so it will need two of you and something to catch the water.I usually put my thumbs over the hole and carry it outside to drain.The water will most likely be dirty so be carefull.
Again, If you have a combi boiler, you will have to top up the pressure when you re-fit the rad as it is a pressurised system unlike the convensional boiler.