Depends what cardio you're doing. Don't go on the treadmill, it's a waste of time. Go walking/running outside, or better yet, swim. What won't do much is going into a weight room and jumping on cable machines, when you should be focused on losing weight and gaining a good base, not isolating a few muscles and creating imbalances. If you can't do a push up, start with planks and work your way from there. I've never met a PT who would say push ups are bad for beginners. Also pretty much all gyms should have an assisted pull up/dip machine, which can help you work your way up to handling your bodyweight doing dips and pull ups. I agree with Johnny about the 5x5 free weight routines. Heavy free weights work wonders for mass gain.