Enter it once. All start up costs are incurred before making money. You would not have any start up costs once you begin making money. You can write off some off your start up costs (up to $5,000) and amortize any remainder. If you go over $50,000, there are different rules. The SBA has a great section, Calculate your startup costs.
Notice in How does starting a business affect my taxes? those expenses reduce your taxable income.
Once you start making money, you will have to pay the Self-employment tax and need to make quarterly Estimated taxes or increase your withholding with your job.
Reference: Small Business and Self-Employed Tax Center | IRS
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