Short answer is that you use carbon dioxide (CO2) to pressurise your keg. The higher the pressure the more bubbles dissolve into your drink until they reach a point no more absorbs. So if you set the pressure at 12psi for instance it will stop absorbing CO2 at about the right "bubblines" for beer. 15-18psi for soda, 10psi for stout.

For more in depth info see this page.

Pure nitrogen and nitrous oxide both dissolve into liquids and when it's poured afterwards cause it to foam in a way resembling the classic Guinness beer with cascading head.

Nitrogen is quite difficult to dissolve into liquid though and needs much higher pressure than CO2. You should have all your ingredients as cold as possible (cold liquid is easier to dissolve gas into generally) then set the pressure at 45psi and either shake the keg or leave it to sit for a few hours (the longer the better) to infuse the nitrogen into the liquid.

If you have a lot of empty space in the keg you will possibly use one bulb just to presurise the keg and need to add more to infuse and then pour with.

As a general rule you want to dissolve one full bulb into the liquid and then use more bulbs to keep the keg pressurised and pour with. To do this shake the keg or leave it to sit at 45psi until the pressure starts to drop and turnning the dial up doesn't increase it anymore. This is a sign that the bulb is empty so TURN OFF THE REGULATOR then unscrew the bulb. Insert a fresh one and then turn the pressure back up to 45psi to pour. If the coffee etc hasn't got the level of foam you'd like then shake or wait till more gas has absorbed and try again.