NOOB ALERT! Help! My object center keeps moving...

and I can’t put it back… So when I try to do a mirror modifier it’s all wrong. How to?? Thanks.

NOOB ALERT!

First thing to learn is how to ask a question. Supply as much info as possible in a clear way to make it easier for people to give you a clear answer. Are you using blender 2.49 or Blender 2.56? The answer will be different. Lests assume blender 2.56.
Use Ctrl+Shift+Alt+C (or the origin button in the toolshelf) to set where you want the objects origin. You can also use Shift+S to set where you want the curser (such as a selected verex) and then position the origin at this curser position.

You could also try selecting your object/vertex/face in edit mode and going to the mesh submenu - go to snap and look at the cursor options.

I don’t know about Blender 2.56 but if you are using blender 2.49 then do this; Ctrl+Shift+Alt+Cthis is to set where you want the objects origin.