Don't panic! I almost failed sewing class in Home Ec. and I have successfully costumed all my shows.
Go to the library or on-line to search for a "look" that you want your costume to have. Print out what you like.
Research your local school and public libraries for simple costume books. There are plenty out there.
See if your costume books have something similar to the costume "of your dreams."
Consult your "network" of friends and colleagues for suggestions.
Send a note to your cast's parents requesting a group meeting before the play rehearsals begin.
At the initial meeting, present your ideas and the resources you've discovered. Admit your inadequacies (if you have them) regarding sewing. Don't be afraid to ask for help!
It's amazing how many parents and grandparents will volunteer. For those who can't sew, suggest they help you by: scouring attics and garage sales, checking out thrift stores, and asking friends and neighbors for "costume contributions."
Ask the group for a costume coordinator; you'll be surprised, you'll get a volunteer!
Don't insist on a realistic look; go for the "impression."
