If Newton's first law says an object in motion will stay in motion unless acted on by an outside force, why does my spinning top stop spinning?
Your spinning top stops spinning because it is being acted on by outside forces. Specifically, your spinning top is losing energy due to friction with the ground and air. That energy is being lost to sound and heat as your spinning top pushes against the air in the room and rubs against the floor.
Instruction: If Newton's first law says an object in motion will stay in motion unless acted on by an outside force, why does my spinning top stop spinning?
Context: 
Response: Your spinning top stops spinning because it is being acted on by outside forces. Specifically, your spinning top is losing energy due to friction with the ground and air. That energy is being lost to sound and heat as your spinning top pushes against the air in the room and rubs against the floor.