You bought in the store is bullet pack is not bullet. If you want to shoot the gun, you need to use bullet pack in Pause Menu(Press "P") to add 10 bullets.
For your help people problem, you try to touch the people then pop-up a window then the window will show what does he need. Then if you If you gather all the things he needs, you can press "Help" button to help him. Otherwise, "Help" button have not any effect.
Shoot the gun:
Help others: