How do I show that I have sold a fixed asset?

You will need to remove the asset and the accumulated depreciation from your books with a journal entry: you would debit the accumulated depreciation, credit the asset that was sold, debit the cash account (I am assuming you received cash) and finally credit you gain on sale of asset - this should be an other income account type. Since your book balance is at zero if fully depreciated the cash received would be a gain.