Why are you wanting to know?, basically you just buy a speaker with it's impedance printed on it - usually 8 ohm, but 16 and 4 are available as well.
A transistor amplifier will have a specification which gives the lowest impedance it's suitable for - usually 4 or 8 ohms, but sometimes as low as 2 ohms. Using a higher impedance speaker will just reduce the power output, using a lower one will probably damage the amplifier.
A valve (tube) amplifier will have an output transformer, this will usually have different taps for different speaker impedances - you simply select the correct tap for the value printed on the speaker. On these sorts of amplifiers it's fairly important to use the correct transformer tap, running into the wrong impedance can damage the amplifier.