how do i make a gun that shoots projectiles at where the player is looking?

Create an item > Click on advanced properties > Check the "Is this item ranged" box > Underneath choose every property you like (You can create custom bullets too but there is a dropdown with arrows and stuff)

Everything else is pretty self-explanatory. Most questions regarding the options you can choose from are answered in the little "?" Buttons next to the option titles.

Hope this helps :D