I always tell people to start small, and work with characters that you really like, connect to, and have passion for. Don't be afraid of making mistakes, because that's one of the best ways to learn. Research and familiarize yourself with different materials, learn what works for what. AND HAVE FUN!
one last thing. where do you find it best to get the materials for your costumes? i know that you can't find some of the fabrics in random art supply stores, but i don't want to break the bank when buying everything.