You can learn a lot about the basics of sewing on various sewing related sites - look at the web sites of pattern companies (simplicity, burda, mccall's, vogue), or check out the American Sewing Guild (
http://www.asg.org) and the Home Sewing Association (
http://www.sewing.org) for links to online sewing lessons.
Another resource is the Greater Portland Area Costumers Guild (
http://www.robewarriors.com) - they meet on Tuesday nights at B Sharp Fabrics in North Portland on Lombard. The members know a lot about sewing and can provide advice and assistance as needed. Attempting to learn everything about sewing on your first costume is difficult, but not impossible. Try to see if you can find a pattern that's very close to the shape you need for your costume. As long as you're reasonably close to a standard size (that is you can walk into a store in the mall and easily find clothing that fits you well) you won't have to do too much fitting of the pattern.
For places to go to purchase stuff, there's JoAnn's fabrics, Fabric Depot, B Sharp Fabrics, and many other stores with sewing supplies all over the Portland area. Open your yellow pages or search the internet.
Good luck!!
