I assume you live in the United States. You are supposed to use the fair market value when donating to organizations like the Goodwill. In my youth I volunteered at an organization that accepted donations, you will typically receive a blank receipt from the organization which you can fill out with the appropriate dollar amounts of the items donated.
If you plan on using this donation as a write off on your personal tax return you will need to do an itemized deduction. 2013's standard deduction was 6100-12200 depending on filing status.
If you plan on using the write off on your business return it would show up on schedule C.