It's best to flush the inside of the keg with CO2 or nitrogen before filling it to avoid your drink touching oxygen (it's only really important for beer, but it's best practise for all drinks). You can do this by injecting CO2 into the open mouth of the keg or though the gas post and pulling the gas release valve a few times. CO2 is heavier than air so imagine your keg is now filled with liquid, the heavier liquid (your drink) goes to the bottom and lifts the lighter liquid (CO2) on top of it as you fill. This prevents your drink touching oxygen while you fill.See a video on gas flushing an open mouth keg hereSee a video on flushing a mini keg with ball lock posts here