SSI applications have narrower online eligibility rules than Social Security retirement or disability applications, so it's worth checking whether you actually qualify for the online path before assuming it's available to you. * The online application at ssa.gov/ssi is only open to applicants who are between 18 and 65, have never been married, are U.S. citizens residing in one of the 50 states, D.C., or the Northern Mariana Islands, and haven't previously applied for or received SSI. Fall outside any of those specific conditions, and you'll need to apply a different way instead. * If online isn't available to you, the other two paths remain fully open: call 1-800-772-1213 to schedule a phone appointment to apply, or visit your local Social Security office in person — calling ahead to book an appointment is strongly recommended rather than walking in unannounced. * Whichever method you use, be ready to document your income, financial assets, living arrangements, work history, and potentially medical information if disability is part of your basis for eligibility. SSA simplified its initial SSI application in recent years, cutting the number of questions from 54 down to as few as 12 depending on how you answer along the way. * Timing matters in a way that's easy to overlook: SSA generally can't pay benefits for any period before your application's effective date, meaning delaying your application doesn't just delay your first check — it can permanently cost you benefits for the months you waited. Applying as soon as you believe you're eligible protects against that gap. There's no fee to apply, ever, at any stage of the process.