Material selection is a step in the process of designing any physical object. In the context of product design, the main goal of material selection is to minimize cost while meeting product performance goals. Systematic selection of the best material for a given application begins with properties and costs of candidate materials. For example, a thermal blanket must have poor thermal conductivity in order to minimize heat transfer for a given temperature difference.
Your first question should be about your design. One main question I ask myself when I design is to ask what the purpose is of the design and what it’s being used for. Second, study the reason why you came up with the design and find out the information you can. What you need to think about are the design specs and what would work best. If your trying to build an RC car with a metal chasis, find a good metal. Thinking aluminum would work may not work. Another thing to think about is Trial and Error. Physically make a model of your design and work from there to find the best suitable materials you can find.
Depends on the design, on the specifications of whatever the design goal is.
material selection is certainly part of the design process.