Frame mounted pump, patch/tire kit with an extra tube, fenders are a MUST if you are going to ride in the rain at all, and a helmet. I'd say get a rack of some sort if your commute is over 2-3 miles as well, because a backpack on your back is going to suck. Save until you can get all of these things and you will be better off.

EDIT: And if you are going to ride at night at all, you need at the very least a rear and front light set. I was coming home the other night, and some kid on an MTB rode past me, no lights, nothing reflective at all. Great way to end up as a human pancake.