The molotov cocktail is the only true "consumable" weapon that is in the game. Using the molotov can be very dangerous if the fire isn't controlled, and crafting a molotov can be very costly in valuable petrol.
A molotov is very costly, requiring 1 petrol, 1 empty wine/beer bottle, and 1 dish towel/ripped sheet. The petrol will be quite hard to find, and because only 1 cocktail comes out of all of these resources, it is up to you whether you truly need it or not.
To throw the cocktail, you need a lighter in your secondary slot and the molotov cocktail in your primary slot. Then, throwing it will cause a 3x3 fire that will spread to any zombies, NPCs, players, floor. The fire will kill anything that it touches, including you. Make sure that you can control the fire and make sure that there is a way for you to escape the fire before you toss the molotov.