What kind of barrel have you got, Bubble? The responses you're getting about gassing and purging refer to the use of stainless steel kegs, usually
Cornelius branded ones which are readily available second hand. The beer is transferred to the keg and can be force carbonated using CO2 gas (from a bottle/tank with a regulator attached) under high pressure, or have sugar added to allow it to carbonate naturally over a couple of weeks. The same CO2 bottle is then used at much lower pressure to dispense the beer.
Your barrel sounds like it's probably a large plastic vessel? Some beer styles (English ales, for example) lend themselves to being gravity poured from a non-pressurised container, with very low carbonation levels, but this is not what "kegging" generally refers to.