Hi, at the beginning of the first level you have no ammo, after you escape the enemies and drop to a lower level there is a weapon to your left, if you grab it you will be able to shoot. Now, if even after grabbing the weapon you still are not able to shoot, then you might have found a bug, if so tell me please your OS and if you can replicate the bug, also the game was made on Windows 10 so I am not sure if it works 100% on other platforms.