Selling a share of a mutual fund is similar to selling stock or other investment property. To compute the capital gain or loss on the sale, you subtract the price that you paid for the shares from the amount that you sold them for. In addition, you report the date that you purchased the shares and the date on which you sold them to determine whether your gain (or loss) was a long-term or short-term one.
For details on calculating the gain or loss on a sale, see Reporting Mutual Fund Share Sales.