http://www.physicsclassroom.com/class/vectors
this may website could help you.If you not learn vector component yet,you could start form lesson 1.
If you did,you can start from lesson 2.
And to determine where the nerf gun will land,you need the initial velocity(take off speed) of the nerf gun.The height from the gun to the ground that you strat will also affect where it land.