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.
Few things look worse than wrinkly satin.
Choose a costume that matches your skill level. It's good to challenge yourself, but when choosing a costume you should evaluate whether or not you have the skills necessary to make it. Start simple and work your way up to the more complex costumes. One thing that never fails to piss me off is seeing a cosplayer who chose a costume with lots of details and accessories, and was just too lazy or unskilled to make them. It just makes you look like an idiot.