Do you have a keg set up with a gas line in, and liquid line out, or are we just talking about a barrel with a liquid tap, and like a screw on cap or something?
If it is the later, I would suggest using the priming sugar that came with the kit, and use the calculator below for figuring the correct amount of sugar for your final volume of beer. You need to boil the correct amount of sugar with the correct amount of water as per the calculator, add the cooled mixture to the bottom of the barrel prior to moving your beer in, and gently stir to incorporate. Seal up the barrel as tight as you can, then let sit for two weeks at about 70 degrees, and 1 week in the fridge to carbonate.
I'm not sure what kind of set up you have, so it's hard to give you advice. The coopers kit should have given you some idea of how to carbonate with the priming sugar, and there should be some UTube videos on the subject also.