in your main user interface while loop check if mouse is clicked
check is there a mouse_ent
check whether it is a pickable entity or not (you can use a FLAG for it, set on level design)
use vec_for_vertex or vec_for_bone to position this item entity to your character hand (even if it is invisible)

on left click check whether the character has an item in hand
if yes, start throw animation of your character
when it is finished after some frames, set a speed to the item
decelerate item speed, apply gravitation, until hits something, or use physx

on right click check whether the character has an item in hand
if yes, detach from character, and set a gravity to the item