Dump all your shit except for personal papers (SSN card, birth certificate, passport, checkbook) sell as much stuff as you can, get 2 changes of clothes, a sleeping bag, foam mat, tiny netbook, phone, charging cable for phone and laptop, poncho, and maybe a 4x7 camouflage (and ideally mylar/reflectorized on one side) tarp.
If you can, put your laptop and phone and charger in a waterproof bag, along with your clean clothes.
Also get a tough little bag for your hygiene stuff so it doesn't explode in your pack. No bar soap, just use body wash or shampoo.
Get shower shoes too, put them in an pouch you can reach from the outside.
Get 2 good lightweight water bottles. Powerade bottles are good short term, just replace them once a month or so.
Consider getting a scrub brush for showering. Get it and as much of your other stuff in pink so no one will steal them.
Don't tie your shoes too tight, your feet will swell up and give you nasty blisters.
Get on your phone and download/bookmark as much info on homeless resources for your area.
Figure out where the Department of Health and/or Human Services is, specifically the food stamps office.
Get the address of a day shelter and get mail set up there. If you're a veteran look for a veterans shelter.
Once you get mail set up at a day shelter, file for food stamps and get set up. Say you're homeless and mentally ill and trying to file for disability.
Then spend your days in the library reading books by NOLO (a law organization) on how to file for social security disability. Spend the rest of the time reading up on mental illnesses, and studying the hobos around you.
Get used to lying to hobos, most of them are crackheads. Lying is a skill like any other, you get good with practice.
Focus on lack of affect (being stone faced or just keeping your face blanks and the muscles of your face relaxed) and lack of vocal affect (not showing emotions in your face).