An Automated Customer Account Transfer Service (ACATS) transfer of stock from one brokerage to another isn't actually a taxable event. It's similar to moving your money from one bank to another. You don't need to report this transfer on your tax return.
However, you do want to keep a few things in mind:
1. Your cost basis (what you originally paid for the stocks) and purchase dates will transfer with your investments to the new brokerage. It's important to keep track of this information for when you eventually sell.
2. If you do sell stocks during the year, those sales would need to be reported on your tax return. Your brokerage should provide you with a Form 1099-B showing these sales.