There isn't an easy answer.
In the short term, social services or mental health agencies or a hospital would take care of you for a little while in the interim with questions asked but answers unavailable.
In U.S. immigration law toddlers of unknown origins are deemed U.S. citizens if found in the U.S. under the "Superman exception" (really!). No exception applies for adults, but a native speaker's accent alone might prove citizenship of cryptic origin by a preponderance of the evidence. Still it can be a mess. See Yu-Ling Teng v. District Director, 820 F.3d 1106 - where intractable hell emerged due to an erroneous birthdate on naturalization papers. When a judge calls your case "a bureaucratic mess of Gogolian proportions" before admitting he can't help, you're screwed.
Once citizenship is established (at least to the satisfaction of the Social Security administration), you could probably get a provisional social security number and from there (1) take a driver's test and get a provisional driver's license, (2) get a job, and (3) find someplace to live.
Marriage could allow an end run around some of this. A minor who seemed to be a U.S. citizen would probably be sent to foster care.