Start out experimenting with sewing in general. Buy a pattern to make some pajama pants and learn how patterns are used, what kind of stitches are used, how your sewing machine works, etc. Doing so will give you a mindset on how to design costume pieces, along with how to refine your pieces (seams, ends of material, hemming...).
A simple sewing machine, some scissors, and a pincushion is a good start. Learning the ladder stitch (hand sewing) is a good idea as well. Everyone starts out not quite knowing what to do, and experimenting with basic patterns will do you a lot of good