The distribution is counted as taxable income until you enter enough education expenses to cover the distribution. If you have a 1098-T, enter it under Education expenses. You will be asked how much of the expense you want to allocate to the credit, and then the balance will be used to make your distribution nontaxable.
If you did not receive a 1098-T because the expenses were for something like high school, which is allowed by many 529 plans, and all the distribution was used for eligible expenses, you don't report it at all.
