The small plastic cap is if shaped like a flying saucer? That's just to keep it closed to the air.
You need to buy a 2 piece bung, usually dark brown rubber with a tan colored plastic piece for the center.
You need to wet the shaft of the tap before using. This will act as a lubricant when you force the tan center piece into the keg. and seat it all the way in.
Priming is 1-2 tablespoons of corn sugar. I just measure and pour it in. Fill with beer to within 1 inch from the top. Wet bung so it'll slide in easier.
Let sit for about 1-2 weeks as you would bottles.
Once opened (tapped) the beer should be consumed. I't not very good even overnight. Some beers may be, but generally not so I just recommend once opened it has to be drunk up.
Once emptied remove the rubber bung and the plastic piece will fall out. Rinse well with hot water...shake...repeat...