Sewing just takes practice. Don't give up, and eventually you'll be good at it. Buy lots of muslin and practice practice practice before you cut up your costume fabric.
Don't worry about your weight, either. It's fantastic that you're trying to get healthy and get in shape, but don't put off cosplaying until you're "skinny enough" or anything. There are lots of plus size cosplayers out there, and if you just want to do it for fun, then it doesn't matter what your weight is.