You can file for any type of Social Security benefit by phone at 800-772-1213 or in person at your local Social Security office (call ahead to book an appointment). For retirement benefits, spousal benefits and Social Security Disability Insurance (SSDI), you can also apply online, in which case it helps to have a My Social Security account.
https://www.aarp.org/retirement/social-security/questions-answers/how-to-apply.html