Modeling help quick plz

I have one problem how do I mirror along an axis I cant remember.
I know its a stupid question…ive just really started modeling though…before I only did very simple modeling and mostly just messing with scenes
dont have much time plz reply quickly:(
if you add a mirror modifier to an object, there will be a series of boxing along the bottom of the modifer with x, y, z on the buttons choose the axis you want to mirror
if you do not see the mirrored object, you might need to go in to edit mode and move the verts untill you see a duplicat set of verts appere, some times the mirrored objet is right ontop of its reflection.

hope this helps and keep blending