Can't add mirror..

I am starting to model a head, and I have subdivided the box and deleted half of it… Now I want to add a mirror, but it wont let me… What do I do wrong?

What do you mean it won’t let you ?
Can you not add a mirror modifier ?
Can you add one but see no effect ?
Ensure to object origin is on the centre line of the model and apply any rotation (Ctrl+A)
Check you are using the correct mirror axis in the mirror modifier

Show us screenshots and supply a link to your blend file

i would guess you have the wrong axses selected in the mirror modifier, or the object origin is in the wrong place.

please share a screenshot in edit mode, showing the object, and the modifiers you have applied to the object.