Let me just say this game needs some work in my opinion. There is nothing really telling you what you are doing so you are having to figure it out yourself. Using the terminal in the first room, it does not explain how to get ammo. Once you try typing it in, it says connection terminated and kicks you out of the terminal. On the table, you have to get your vacuum and then you have to go interact with the thing on the wall.
Once you go outside there is a crate that contains one ammo. You only get two ammo in the backpack vacuum thing and if you left click you will end up shooting it and you not even notice it or know it.
\Your goal is trying to survive and clean up the backrooms however, it is not as easy as you think. If your teammate runs by the toxic barrels they can shrink and it can end up killing them. There are also monsters you have to try to kill and escape by but due to the ammo being limited it is hard to do.