Ok ladies, I too started about a month ago going to the gym and trying out different aerobics clsses such as: zumba, awesome abs, core & more and occasionally working out with my brother who is a personal trainer. Everything that they tell me or show me at the gym I check with my Brother because he eats, sleeps, breathes nutrition and working out.
So I would say from what I have learned for the mid section is #1 thing PLANKS and SIDE PLANKS, they are killers but for a good reason THEY WORK. When I first started I could not even hold it for 20 seconds and now I am holding them over a minute. They will also make you instantly sweat!! Side planks are still very difficult for me but I keep practicing.
Also, bicycles... laying on your back with your legs up in the air like you are riding a bike while raising your head and shoulders as much as possible. Even holding your legs in the air, cross your ankles and do crunches will work the lower stomach.
Oh and if you don't know what a plank is.... You get in push up position but instead of being on your hands you rest the front of your body on your forearms, the only thing touching the ground are your toes and your forearms...a side plank is similar except you are only on one arm and your legs should be stacked so only the outside of one of your feet and one forearm are touching the ground (good luck with that one)... hold them for as long as you can, rest 10 seconds, do it again, rest, do it again.... as many as you can before you have to quit for the day.