I wanted to duplicate an eye and bring it to the other side to make a self-portrait, but I have no idea how to. The head and the eye are both in different layers, just so you know. (If that's of any importance), but please help.
Select the Move tool (1st tool in toolbox)
Make the eye layer active
Depending on what the eye looks like, you may want to use Edit > Transform > Flip Horizontal for symmetry.