On this page of the Disco Elysium guide, you will learn how to get the Yellow Plastic Bag, and what you can use it for.
The Yellow Plastic Bag is an extremely useful item, especially at the beginning of Disco Elysium, as it will allow you to earn extra money. You can get the bag right at the beginning of the game.
Exit the hotel, walk past the bookstore, and head to the location shown in the image above. The Yellow Plastic Bag lies right next to a man leaning against a wall.
Equip your character with the new item (it doesn't matter in which hand), and you'll notice that you can pick up bottles from the ground, as well as interact with some trash cans. You can sell the items you acquire this way to gain additional funds.
For more information on how to make money in Disco Elysium, check out the Money - how to earn? page.