First you must be certain that there are no laws in your town that prohibits trash picking.
Next you have to have a vehicle (preferably a truck, just for larger stuff.) You do need some storage space to keep the stuff you find until you sell it. Wait until after dark to start picking; that way most folks have their garbage cans out and less people will see you.
Choose an area that you know has garbage pick-up the next day. Only stop when it looks like it’s something that will sell. Some things to look for are collectables, antiques, tools, nice wood furniture, good dishes, etc.
Okay, now that you have a nice stash of trash, it’s time to sell it and make some money.
A couple options I’m familiar with are flea markets and yard sales. Flea markets will charge you about $10 to $25 to set up for a day. If you have some nice things to sell you can make up your rent money selling one item and people are usually willing to pay what you ask.
Yard sales are another way of selling what you found. Some towns require you to get a permit, some charge a small fee, and some towns you need both or neither; it just depends where you live. You might get less for your stuff having a yard sale but it was free anyway. The best thing about picking and selling trash is, there is an endless supply; you can make money every week.