Start material selection with the application.
The first question is how the part will actually be used.
Define loads, temperature, chemical exposure, environment, service-life expectations and appearance. These requirements provide a framework for narrowing the material family.
Next, review geometry and the manufacturing method. Wall thickness, dimensional requirements, joining methods and machine conditions should be considered alongside the material.
Finally, consider supply, variability and validation. Define application-relevant tests and acceptance criteria for candidate materials, and record the remaining uncertainties.