On this Disco Elysium guide page, you will learn how to get a flashlight and how to use it.
The flashlight is an extremely useful item that will help your character explore dark rooms, find hidden items and clues necessary to complete certain quests.
You can get the flashlight at the start of the game. Exit the hotel and approach Kim's car.
Look inside, then open the Toolbox - you will find the Flashlight. Just select the dialogue option highlighted in the image above, and the item will go into your inventory.
Before you enter a dark room, equip your character with a flashlight. You can control the light with the mouse - the hero will direct the flashlight to the place where the cursor is.