You could start out, as some suggested, just decorating old pieces of yours and altering them. If you feel ready for the challenge (not really that difficult if you've got an eye for shapes and how they fit together), learn how to sew. An easy project I can think of off the top of my head would be a cape or pirate pants (you know, the wide thingies with elastic in the waist). Kind of cliché, but possible to do. Do not use expensive fabrics. Oh, and old curtains are your best friend. I adore curtains; often get them in thrift stores, wash them and use them in my sewing projects. For instance, I recently made a kimono out of a pair of old curtains from the 60's or 70's. As curtains, they must've looked absolutely horrible, but the kimono looks cool. =) Anyway, recommended reading:
http://www.gothfashion.info/beginsew.php