I am assuming you are referring to the small disposable 8g CO2 capsules, if so here goes.
When you initially fill your keg with beer, you should also be adding a small amount of sugar to prime it.
About 100g of sugar is correct for a 5 gallon keg of ale. Lager doesn't work in plastic kegs as you can't get sufficient pressure in them to make it fizzy enough.
Once your keg is full of beer, and you have added your 100g of sugar leave it in a warm place for about a week, this will get the yeast to work on your sugar, producing lots of CO2 which will fill your keg up and pressurise it. At this stage you do not need your CO2 capsules.
After about a week move your keg to somewhere cooler and leave it to condition for as long as you can, some people have patience, some don't, a rough guide is 1 week per percentage point, so if it's a 5% beer it needs at least 5 weeks to condition.
When you start drinking the beer it should come out of the tap with a lot of pressure, if it doesn't you have a leak somewhere, usually around the lid seal (don't overtighten the lid and apply a little vaseline to the washer)
As you continue drinking the beer gradually the pressure in the beer will reduce. The purpose of the CO2 capsules is to prevent any oxygen getting into the keg, this will happen when the pressure gets so low that air gets sucked in the tap when you open it.
To prevent this you have to kind of guess when this will happen, and just before add a capsule of gas. It will release all its gas into the keg in a couple of seconds and then you can remove it.
Longer term you may find S30 cylinders are a better investment than the little capsules, that will depend on how much you brew though.