The previous reply from VolvoGirl is mostly correct for most people, but omits some details and variations - of which there are many.
I suggest using the IRS Interactive Tax Assistant, which walks you through a series of specific questions to determine whether or not your social security income is taxable, and if so, how much.
It is available at
https://www.irs.gov/help/ita/are-my-social-security-or-railroad-retirement-tier-i-benefits-taxable
You start the process by clicking the "Begin" button at the bottom of the screen.
