Welcome. By TC 18, I wasn't sure if you meant v18 or v2018, but the tool works is pretty much the same with a few extra options in 2018.
There is a specific tool for what you require, Assuming you need a copy. If the objects are just 2D, you could simply rotate around one axes, but this will not give a copy.
The Mirror Copy tools location depends on the configuration set in tools - customise - options page (this sets up how TC menus looks on screen), and Mirror Copy may be in the Edit Menu, or somewhere in the Modify menu, it is also on a toolbar but again the location may vary.
To use, please see this TC v20 help page.
(I couldn't reach the TC v18 help files on the net).
One thing that help file doesn't make clear, is that when drawing the mirror line, you can snap to the Grid or other object, thus ensuring the mirror line is exactly where one requires it. One can also turn on orthogonal snap, (or hold down shift key) to restrict the mirror line to horizontal or vertical.