The details will differ somewhat from person to person. But in general:
Start calling homeless shelters around you, see if you can find one that has space for you.
Homeless Shelter Directory helps you find homeless shelters and food banks.
Help When You Need It helps you find food pantries, stores that accept food stamps, assisted living facilities, domestic violence shelters, homeless shelters, hospitals, mental health treatment, substance abuse treatment, free clinics, V.A. facilities, women's clinics, legal assistance or financial assistance.
National Coalition for the Homeless has various resources, depending on when you expect to be homeless.
Make sure you store addresses now in your phone, so you don't have to worry about remembering that later.
Generally libraries are safe havens. Free computer access and you're sheltered from the elements. So make sure that you know where the nearest libraries are.
Consider learning how to panhandle:
Most watched youtube videos:
If you've been active with your reddit account for a while, you can go to /r/Assistance and submit your problems there and either ask for help or post wishlists. Read the sidebar before posting or else your request will be deleted, keep in mind that you'll be asked questions to validate your request. Don't expect to receive anything without considerable intrusion to your privacy (it's nothing personal, they have to do this to weed out the bad people). Also watch out for scammers. Don't give anyone your bank logins.
Join these subreddits:
If you have a car, join this subreddit:
And ask specific questions there. Those are the experts you are looking for.