I gained weight due to having thyroid issues. I'm taking thyroid meds now (1/2 grain of Armour daily) and can join the land of the living again (really, I was so bad I had trouble thinking and moving around for a while!) but I think it will take a while to lose the weight.
Anything processed -- meaning I haven't prepared it myself -- is highly suspect to me. It's a pain, but it's true. You never know what is in prepared foods these days that will mess you up: hydrogenated oils, corn products, soybean products, and artificial sweeteners all add bulk and calories to an item at low cost to the processor, but with bad results for you.
Eat fresh and local and much as possible, and that will help tremendously.
I drink soda only as a treat and maybe once a month. Otherwise it's plain old water, tea, or coffee for me. No sweetener, but I'll add some goat milk (if I can get it) to black tea.
It takes a couple weeks to wean yourself off the sweetened drinks and highly processed foods, but once you do you won't like them anymore and will instead find yourself craving fresh, wholesome foods.
I don't sweeteners completely out of my diet. I will add agave necter, honey, and real sugar (in that order of preference) to baked goods, etc. if necessary. I'm lucky because I've never had a really bad sweet tooth.