Choose fabric that suits the costume. For example, don't use flimsy or stretchy fabric if the costume is supposed to be a tailored jacket and pants. If you're making a pleated skirt, make sure that the fabric is the kind that will take a pleat before you buy it. If the costume's supposed to be skin-tight, do yourself a favour and get stretchy material. The problem with shiny fabric is that it shows every single wrinkle and fold, so if you haven't done a perfect job of fitting your costume it will be glaringly obvious.
Satin is also very hard to hem and often ends up puckering horribly.
If you're a relative newbie, do yourself a favour and choose a fabric that won't show every little mistake. Another thing about satin is that not all costumes are suited to shiny fabrics. If it's clear from your reference pictures that the character is wearing plain matte fabric, then it's pretty dumb to make your costume out of satin. Matte fabrics are also far more forgiving to those of us that don't have perfect figures. And for goddess' sakes, if you're using shiny fabric, bring a small steam-iron to the convention.