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Do you mean decied what impedance you require, this is down to the signals you are routing, or do you mean the stack up?
If its the stackup talk to your PCB manufacturer.
Fundamentally- impedance varies with following parameters:
Trace width (inversely proportional), layer thickness (inversely proportional), separation from the nearest reference plane (directly proportional) and dielectric constant (inversely proportional)
1. How to select the impedance- really depends on the circuit requirement- example most of single ended signals (well most) need either 50 ohms or 60 ohms impedance. While most differential signals need 100 ohms differential ohms impedance (90 ohms in the case of USB).
2. How to achieve a desired impedance- by varying the above parameters. As Marce suggested, talk to your PCB fabricator and give him the requirements and the layer count- easiest way of making an impedance controlled PCB
The starting point for most PCB stackup design is determining what impedance or
impedances to use in each signal layer. A number of impedances have been used for
controlled impedance PCBs. Among these are 62 or 65 ohms for PCI buses, 72 or 75
ohms for video signals, 50 ohms for ECL and high speed CMOS, 28 ohms for Rambus
and an assortment of differential impedances for various differential signaling protocols.
The most common impedance found in multilayer PCBs is 50 ohms.
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