"I'd like to tone up my butt, thighs, and stomach, but I don't really know what to do besides run the treadmill and do some sit ups."
There are two ways to change your body--lose fat, or build muscle, or both. "Toning" just means a person who has little fat on top of their muscle. In order to achieve the toned look, a base of muscle mass is required.
If you lose weight without gaining/maintaining muscle, you will in general become a smaller version of the shape you currently are. If you want your butt, thighs, and stomach to look more "toned" you'll need to build muscle and lose fat.
Here's a decent routine: https://www.muscleandstrength.com/workouts/phul-workout
A lot of women like this one: https://toptalkedbooks.com/amzn/1936608642
Really any beginner lifting program will work if you stick to it.
Contrary to the other reply, I've lost about... 35-40 lbs since I started working out 8 months ago and I was in your position when I started. I could hang on the bar for a solid 5 seconds if I was lucky. Now I can do 3 pull-ups, which maybe isn't a lot but I'm still damn excited. I just do p90x, and I started doing pull-ups with a chair (the routine has you doing them twice a week). I eventually moved on to negative pull-ups which really helped me in being able to do real ones.
As for the butt, it's something I need to work on but I hear people mention this all the time: Strong Curves
Amazon , and from what I understand it can also be found at Barnes and Noble and Walmart!