Or just ditch the weights and do pushups. No bench needed! If you can't do a full push up yet, you can do modified ones with your knees on the ground, or do stability ball pushups. Flop onto the ball on your stomach, and "walk" out with your hands to where you can balance and do 10-12 pushups. Today that may be your upper thighs. In a few weeks, maybe your calves. As you get stronger, you'll be able to go farther out- by the time you can do them with your ankles on the ball, I guarantee that you can do "real" pushups on the floor!
I would also add flyes on the stability ball. I don't think dropping out chest exercises is a good idea. As you lose weight, most women tend to lose top down. And that means looking saggy. Good musculature helps
I just re-read Northern Belle's post. Her version of a stability ball pushup is an excellent exercise, but it it a lot harder because your hands are on an unstable surface. IMO, you need to be able to do at least modified pushups on the floor fairly well before you try that version. If you have back isssues, try with your stomach on the ball and your hands on the floor.
Mel