You can apply for retirement benefits at your local SSA office, by phone at 800-772-1213, or online. You must be at least 61 years and 9 months old to apply. You should not apply more than 4 months before you want your benefits to start.
More information can be found on the SSA website. SSA has an online tool that you can use to find out if you could be eligible for benefits from any of the programs that Social Security administers.