Hi there,
Cut the sugar. These days, sugar is added to almost every edible product, so you have to learn to read the nutritional value labels. Do not even consider anything that has more than 15% sugar. Give up all sodas, snacks, chocolate etc.
Do not eat ANYTHING after 6 pm. 5 pm is even better.
If you are going to eat something "forbidden", eat it early in the morning.
Excercise. But it has to be something you enjoy, or it will be a torture for you. The simpliest, safest and very effective way to excercise is... to walk. It needs to be fast-paced and last at least an hour, though.
Do not lie to yourself. Never try to excuse yourself with "Yes, it is very sweet but has lots of minerals and fibre". Say "I failed today" instead.
Librarian