How do I select an individual face in the geometry editor?

I’ve rarely been able to do this and I have no idea how this works. I’m in the geometry editor with a “box” and I’m trying to select a face on the box so I can extrude the sides on it, but it doesn’t fully select the face of the box. I’ll “select” a face, but it doesn’t let me change to the extrude option. I don’t understand what I’m doing wrong. Is there a setting or something I’m missing?