Ok this is a stupid question but...

I’m modelling a face, how do I get it to have a mirror copy so I only have to model one side?

EDIT: Nevermind, figured it out. Make a copy of the object, and set its x scale to -1.

Easier way is to use the Mirror modifier in the Edit Buttons section (F9). Before you even start modeling and just have your plane or whatever you’re using, line it up in front view on the Z axis and mirror over the X. The other side is mirrored as you edit so you can see how the symmetry looks while you model.