Aligning vertices to centre I am new to Blender.

I am making a character using the mirror technique, however there is a hole in his head.

I understand that there is a single vertex that needs to be aligned in the middle to fix it, however if I do it by hand it will not be perfect.

How do I align this vertex so it creates a solid head?

In this case, I imagine it is quite simple as it is perpendicular to the X axis, however if it was not, how would I fix it?

Many thanks
You can get there quite easily using Vertex Snapping.

Enable Snapping > Vertex, and use the Move tool to align it to other verts.