What do you mean by "selecting Plastics" ?
1/ the plastic is choosen for the properties of the part :
- low cost : PE, PP, PS (HIPS)
- stiffness (glass fiber ou carbon charge),
- Temperature of use : PA, PBT > PPA > PPS > PEEK,
- Tribology : POM with or without PTFE charge and Silicon,
- ...
For the properties of all material, you need experience, good memory ... and also this website www.ulprospector.com with all the data sheet.
2/ When you have the part and the material, you need to know :
- how to demould the part (natural / slides / lifters ...)
- how to inject (where, how many points ...)
- how to eject the part.
And then, you can do an flow analysis with moldflow, moldex3D, Cadmould, Visiflow or Solidworks Plastics, but for good results you need also good parameters for injection (time), holding pressure / time (very important for shrinkage) and thermal solution.