In brief,
You file your first schedule C in the year your business is active -- when you are actively engaged in ongoing business activities. If you have startup expenses from years before the business was active, you list them in the first year the business is active. Depending on the amount and type of expense, it might be fully deductible in year 1, or partly deductible in year 1 and partly amortized over 15 years. Equipment is listed for depreciation when you place it in service, regardless of when you bought it.
If you have startup expenses in excess of revenue, you create a loss, which is deductible against your other taxable income (you can deduct a loss from a side job against wages from your main job, for example). If your loss is more than all your other income, you have a net operating loss which you can carry forward to the next tax year.
If you have startup expenses before you begin ongoing business activities, you can't deduct them. You save them until your business becomes active and ongoing. For example, if you are a freelance graphics designer and you buy supplies in November 2021, but you don't list your business on Upwork and start advertising for clients until January 2022, then you business activities started in 2022 and you list the supples on your 2022 return as startup expenses.
