Most of the time it will be the fair market value (FMV) of your item. However, if the amount you originally paid for the item is less than the FMV, you must use the original cost of the item.
You can get the FMV by looking at similar items for sale on eBay, Craigslist, or at your local thrift store. When you use ItsDeductible, it will assign a FMV to most items that are commonly donated.