Hey, CowGirl, I am so sorry about your mom. That's gotta be very hard, and not easy for you to try to make changes with having to go through the stress I am sure you're dealing with! Great for you for wanting to make those healthful changes!
About getting better health--don't try diets. Getting on diets implies they are something you get back off of, and they usually change too much too fast. You need to make lifestyle changes--things that you change over time for your health and well being. What do you eat/drink the most of that is worst for you? Try phasing that one thing out and replacing it with something healthful: Soft drinks are the worst, because they are really just empty calories. If you drink a Coke a day, say, replace it with V8 juice or just water. Or if you snack on potato chips or Doritos in front of the TV, try changing to carrot sticks or try to change away from eating while you are doing something else altogether (don't eat while reading, watching TV or sitting on the computer--eat for the sake of eating and gaining nourishment and give your whole attention to it. It's so easy to throw on 1,600 calories from chowing down on a whole bag of Cheetos in front of the computer and not even really notice you're eating them). If you love Cheetos and can't live without them, have 14 of them by themselves and pay your whole attention to eating and enjoying them, then get on the computer.
Make changes slowly--don't all of a sudden change everything you eat and the way you eat and expect it to stick. Change one thing at a time, starting with the worst offenders, then, when you're comfortable with the change, try changing the next worst thing.
About exercise--you might try walking and jogging your horse around a pen or arena or field. You can also do things like jogging places instead of walking, climbing gates instead of opening them, climbing stairs if you have them, etc. I try to climb our set of stairs ten to 15 times a day, when I can.
Joining Curves can be a good thing, but you have to make the commitment or you will be out a lot of money. Is there anyone else who can go with you to keep you going? It's always good to work out with a buddy, because you can keep each other motivated and hold each other accountable. Or maybe you can do that with someone you meet at Curves. You might ask the people there if they know of anyone who might be interested. You can schedule it right after your GED classes (good for you for going back to do that!!), so it's on your way home and a stress-reliever from studying.
You are wanting to do the right thing for yourself, congrats! Good luck on getting yourself in a good place! :)