If you are on w2, your tax is deducted automatically

from each of your paycheck based on the information you provide. If you are not on w2 or have significant income from other sources, you may estimate how much you need to pay and pay it quarterly to avoid any penalty.

On April 15 of each year, you need to figure out how much you have overpaid/underpaid your tax for the previous year. This is called file tax return.

This is for the federal tax in the US.