The important thing to keep in mind when you're a beginner is to pick a SIMPLE costume. Don't jump into something that requires lots of fitting, elaborate trimings, tons of seams or fancy expensive fabric. Pick something simple that consists of only a few straight seams and works fine with basic, inexpensive fabric that you can get for very little money.
Look at all the characters you've seen in your favorite anime shows and pick one whose clothing has the simplest lines. If the character is wearing just a t-tunic type outfit, that's easy. If the character has this fitted to the body fancy bodice with hundreds of colors - wait until you've learned more about sewing. There's nothing worse that attempting to tackle a project beyond your ability to frustrate you enough to quit before you've started.
For Japanese style clothing, this could be a happi coat (it's just a bunch of rectangles) or a kimono top. Or, you could just start with sewing accessories for something you've already purchased.
Simplicity has online sewing training:
http:// http://www.simplicity.com/index.cfm?page=simplicityClassroom.html, and the American Sewing Guild has a great list of links to all kinds of sewing info:
http://www.asg.org/html/links.html. Also The Home Sewing Association
http://www.sewing.org has excellent sewing basics.
In addition, don't forget to attend all the cosplay panels at Kumoricon! Andk, if you attend KumoriCon meetings, find some cosplayers and ask them about sewing.