What type of workout is best for someone who hasn't worked out in MANY years?

All my life (up until 4 years ago) I have always weighed 150. I had a baby 4 years and during my pregnancy my weight went up to 181. I haven't lost ANY of the weight and to be honest, I've tried numerous times but always failed. I would like to seriously lose weight now (I saw a picture of myself and was completely mortified), what is the best workout method for someone who has never worked out? Does walking on a treadmill really help the weight come off?