To get just foam in your glass is not fun!

There is a balance between not enough and too much foam head on a beer glass, sometimes a nice and frothy head of foam is the perfect visual enhancer for a cold mug of beer like in the picture above! But sometimes there is almost no beer in the mug, just foam!
When serving beer from a tap there is two CO2 pressures to consider: The storing pressure and the serving pressure. The storing pressure is the pressure in the keg and, if given enough time, the pressure in the beer as well! Just pushing a lot of CO2 in a filled-up keg will give you a high pressure at the top of the keg but no CO2 in the beer unless you let the CO2 diffuse into the liquid for several hours (or shake the keg - my method!!) The beer will be too sweet and flat until the CO2 acidity has been incorporated. But generally, when you have a keg of beer you can assume that the keg and the beer inside is at the same pressure with thoroughly diffused CO2.
Each beer style has its optimal storage pressure to match the desired taste and mouthfeel and this is what you want in your glass. Often this pressure will be too high to just pour out of a tap and here enters the art of the serving pressure! You will need to lower the pressure of the beer, either by letting some CO2 out of the keg at the time of serving or by using a very narrow tube between the keg and the tap,in the order of 4.5mm/1/4" inner diameter. That tube needs to have enough internal resistance for the pressure to drop sufficiently and therefore it has to be rather long, in the order of several meters/yards. And the length will also be different for different beer styles! This balance can be quite challenging to achieve to perfection, but a more general tubing length is much easier. And with some finesse at the tap is will be possible to pour a perfect draught! I have even got Perlick Creamer taps, the handle can be pushed back after filling the glass with beer and it will then release a foamy head, perfectly!