Increasing Stamina

I work out in the gym primarily to lose weight but also to gain more stamina to keep up with my very active toddler. I hope to replace my fats with lean muscles through lifting weights like how taking cell mass supplements would do. I usually work out on the days when my son is in school, that way I would have more time to exercise and concentrate on what I am doing. But I could also do it on non-school days because on those days the gym provides a babysitting service where I pay people to watch my son while I do my work outs. So there's no excuse anymore for me not to go to the gym.