There are some free image programs out there, but you don't really want an image program for designing.
Adobe Illustrator is a good program, but $200. If you are a student you may be able to get a discount on the price for getting the Student version (limits you to not useing the program to make money from what you create. how they track that, I dont know).
If you are useing Mac, there is CADintosh, a $35 CAD program. I use CADintosh for all my plans, but am looking at getting a newer version of illustrator.