If you do not need sell any RSU, you do not need to pay any tax yet.
When RSU is vested, the company sells the part of your RSU and all gain/paid tax by sold RSU are in W2. That’s it.
If you sell RSU, you need to know the base cost at the time of vested. If you sell your RSU, the sold price minus this base cost is your gain, which you need to report. This base cost is usally shown in transaction history or form.