I find the easiest garb to make is a simple tabard.
http://lh5.ggpht.com/_369bYz0cioo/SeKdu ... GP5936.JPGHere's a picture of one of the easiest ways to make it. This picture has two colors of fabric, but it really only needs one.
1. To make it is simple. Cut out a rectangle about as wide or a few inches wider than your shoulders. Make it long enough to go from around your knees, over your shoulder, and back down to knee-level.
2. You know have a very long skinny piece of fabric. cut a head hole in the center of it (I'd guess about 7 inch diameter, but you can make it bigger)
3. Now put it on with the long sides going down your front and your back. Belt it at the waist with a belt or rope.
You know have a basic tabard! good job! I'd recommend hemming it somehow, either by folding the edge over a little bit, or by putting a zig zag stich along the edge. You may also want to sew around the neckhole to reinforce it, otherwise it may stretch/rip.
If you want to go fancier, you can jazz it up by making it different colors (like the picture) or you can sew/embroider some design on the front. You could also trim the edges with ribbon or store bought embroidery or something. Have fun!