Welcome! I like the quote, as cliche as it is "Today is the first day of the rest of your life."
Joining a gym and making a commitment to eat better is the first step to making a change.
I would honestly recommend talking to a registered dietician, if you want to learn how to eat better, and what to eat. It helped me immensely figure out what was lacking in my diet, and what I needed more of. I too have a thyroid disorder (as in, I don't have one anymore). Also talk to your doctor about your thyroid issues, and what foods you should avoid that can interfere with healthy thyroid function.
By "vitamins", do you mean weight loss pills, or something like an Omega-3 supplement, iron, calcium, etc? Or something more like like phen-fen? I found I don't have enough iron and calcium in my diet, and after talking to my doctor, I take a daily iron supplement, along with a calcium supplement. However, everyone is different and can't have vitamins and digest them properly.
Any who, good luck! People are here for support and often have great ideas about overcoming obstacles, plateaus, etc!!