You say it's store-bought beer, but you don't mention that you have a co2 tank and pressure regulator.
If you don't have these, you need to get them to provide pressure to push out the beer, and to keep it carbonated until the keg is empty.
If you do have these, increase the pressure on the regulator until you get a good pour. Try to find out the recommended pressure for your beer style. Raise the pressure slowly over a couple of days, otherwise you may overshoot, leading to a foamfest.
Ex fermentum, scientia.