Help my dress won't zip up!

So last year about the month before I found out I was pregnant I tried on this dress and ordered it thinking I'd be able to lose weight before my brothers wedding. Now 4 weeks after I had my baby it doesn't zip all the way up starting around my bra line. The wedding is last day of March. Im not one to workout with weights so I don't know anything about that, I've always only done cardio. I don't feel cardio will make a difference in my back. Once my doctor gives me the ok to workout what should I do to get the dress fit?