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.
Costume Con - Kili in a dress
Atomic Lollipop - Come visit me at the Make-Your-Own-Button free craft booth!
Polar Chill - check out my costume DIY panel and workshop
FanExpo - Iron Man Mark VII version 2, Pacific Rim Jaeger pilot?
FanExpo - fem!Iron Man Mark VII - The Iron Man Armor Project
Atomic Lollipop - Vanellope von Schweetz; raver!Sailor Mars