You can do several things:
You've aligned your mesh with the global axis but your object doesn't seem to be (the mirrored mesh is tilted), it's a bit disturbing, so in Object mode, apply the rotation (CtrlA).
By default the origin of the object (small orange dot) is the center of the Mirror modifier and as we can see the mesh is far from it for the moment, ...
The Mirror modifier in Blender works based off the objects origin. If you want to mirror the object as a whole you will need to go into Edit Mode and move the object off of it's origin point. Then when you use the Mirror modifier it will mirror the object on the chosen axis based on how far from the origin it is.
You could do it this way:
Create a plane, in Edit mode scale it and move it away form its origin:
Give it an Array modifier, enable Object Offset instead of Relative Offset, create an empty at the same position as the plane center, choose it as Offset, rotate the empty 60° on Z:
Move and edit the mesh in Edit mode:
Extrude it, etc, when you'll be good, ...
There's no such thing as using the world origin in a modifier. Modifiers either use the current object's origin (which you can set to anywhere you want) or another object's origin (which you can also move anywhere you want.
In either case you can do it so you effectively have your modifier acting from the world origin.
Just add an empty plane or anything, and then move it close to the body where you want to see the mirror. Then add a mirror modifier to your body and add a target to that empty. That's all.
You can also move it towards the mirror direction. In your case, that should be X.