Autodesk Maya 2018 Basics- MIRRORING- Duplicate/Duplicate Special/Mirror
3 Techniques to Mirror an Object- Duplicate, Duplicate Special, and Mirror
For ALL 3 methods, it's important to HOLLOW out the FACE side of the object that you WANT MIRRORED that will lie on the vertical Y-axis.
For Mesh menu to Mirror Method, it's less important as there is an option in MIRROR settings to "CUT GEOMETRY". However, I feel as though it's a good general rule to just hollow out the FACE where you want the MIRRORING to begin.