Nope, don't wait--do it now. You'll be building muscle underneath fat, and, for me personally, I've found it helped with keeping me firm as I lost weight. I don't have a lot of extra skin like some people who've lost significant amounts of weight.
You know, your body often changes after you have a baby so even though you may have been 120 lbs. prior to baby, you may not be able to get that low. Hopefully you can, but just keep that in mind and don't get disillusioned. Plus, just ask the Ladies Who Lift--140 lbs. of toned body weight can look a lot better than 120 lbs. of untoned body weight. Many people are the "flabby thin." I started wearing a size 12 when I weighed 190 (I'm 5'9"), which I attribute to weight training. Mrs. Jim weighs, I believe, between 145-149 pounds, is a size 4/6, and I think is your height.
Start out lifting what you can and then work up from there. This is a great site for exercises and information:
http://www.stumptuous.com/weights_index_revision.html