Enter edit mode, select the whole mesh, in the menu choose "mesh" -> "symmetrize"
Then select the correct "Direction" in the Redo panel (the pop-up panel with the settings for the tool)
Make sure your objects origin is in the centre of the object as this is where the symmetrize-ing will be based.
Blender docs: symmetrize
welcome to BSE, you need to work more in the topology, that spike is an "ngon" that got triangulated by the sub-div modifier, for working with surf you need to have a decent quad topology to generate the shape you want.... you small objects seems not merged to that ngon, keep those separated like that...
Your topology is probably not good, here is how you could do your object:
Create a 32 vertex cylinder and a 16 vertex cylinder:
Rework the topology:
Give it a Subdivision Surface modifier, add edge loops to sharp the edges:
Only apply the Subdivision Surface modifier if necessary, don't if you don't need to.
The reason this is messing up your shading is because Blender does not like the topology of your mesh. Blender favors Quad based topology. When you have a shape that has more then four vertices, it is called an Ngon; And lets just say blender does not like Ngons.
In order to fix this, I'm afraid there's a lot of work in it for you. there are a few ways to ...
I'm still new to 3d modelling but i believe the general rule is that for sculptures you always need to retopologize the mesh. If not, the 3d model will lag or crash the computer system when animating or moving it because the mesh is so heavy in topology.
UV-unwrapping the sculpture also becomes impossible to do by hand as there's thousands of different faces ...