Step 1: Don't stress out. First off, stress only makes you tend towards weight gain. Second, it sounds like this is something you're going to be stuck with for a while, but it isn't your fault. Knowing that it's a thyroid thing, I mean, you definitely need to remember not to blame yourself. I did that for almost seven years. I'd get depressed, eat more, get fatter, then beat myself up on the inside, none of which is healthy. Accept it, deal with it, and learn to love yourself a little more each day. It isn't very hard - you have a wonderful body, no matter what it looks like! Curves are easily twice as beautiful as bony twiggy bits. It's your inner beauty that is really going to shine through, anyway, so take a few moments out of your day to cultivate that, first.
If you really are intent on working out though, kudos for doing situps instead of crunches. I do crunches. They'll keep you fit, but not quite as chiseled as situps would, so you're already a step ahead of most people. The biggest, most vital thing I can tell you here is to keep going. Once you've done situps about a week in a row it'll become a habit - just do not stop, ever. Do not take a day off, do not use being tired or crampy or overworked as an excuse to skip a day. Once you do, it'll form a habit and it will be twice as hard to get back into things as it was to start them in the first place. I know. It's pretty easy to work your butt and legs. Taking a long walk every day will make a big difference. I do the "electric chair" thing, where you sit against a wall at a right angle, like you're sitting on an invisible chair. This is an amazing workout, and you'll feel it in just a couple seconds. Don't feel bad if you can only make it a minute or so, that's all most people can do. Do about fifty lunges or squats while you're at it. Those are a lot easier to show than to explain in words, so I'd do a quick Yahoo search for pics if you're interested.
Overall, I think you'll be just fine with whatever you decide. Just remember not to give up on yourself and your goals, and you'll be not only sucessful but a generally beautiful person. Best of luck!