What are the biggest newbie mistakes I don't want to make?

Hey everyone! My doctor put the fear of god into me yesterday and I'm reaching out for advice.

Long story short, I haven't had insurance for most of my adult life and I've been chronically ill with no idea why for many years.

Last weekend I did the nasty 3 hour fast thing and my doctor tells me that my results are not great. She said it's possible it's too late and I'm already diabetic, but that we'll try to treat me as if it's still pre and check again in 3 months. No idea if she's just trying to make sure I get my ass in gear, or if that's as bad as it sounded to me, but either way I don't feel like I have the luxury of taking my time to figure this shit out.

So I want to know what newbie mistakes you guys made, like eating something you thought was safe and it turns out it's not, or doing the wrong thing with exercise etc. You don't know what you don't know and until yesterday diabetes was not on my radar so I'm flying blind here.

Also if anyone has any links to favorite recipes, I definitely wouldn't object to those showing up in my comments.

Thanks in advance everyone! Now I'm off to do a bunch of research.