you want to make it a tool or something like welded to you in fps? for a tool you would want to know UIS or CAC to detect input, loading animations; and remote events. after firing a remote event you want to use a body mover or raycasting to create the bullet, you would want to know region3, magnitude, touched, or use the raycast to use as a hitbox.
sources:
https://developer.roblox.com/en-us/api-reference/class/UserInputService
https://developer.roblox.com/en-us/api-reference/function/Humanoid/LoadAnimation
https://developer.roblox.com/en-us/articles/Remote-Functions-and-Events
https://developer.roblox.com/en-us/articles/BodyMover