I recently started a new diet myself, not because of weight, but because of both emotional and physical health concerns. Here's an info dump on some of the stuff I learned.
Eating a healthier diet isn't like pouring your weight out of a bottle until its empty. Most fruits and vegetables are great sources of natural sugars, and sugars are usually what gets processed into body fat as stored energy, along with carbohydrates, which are rich in foods like wheat, potatoes, and rice (Sugars are actually very basic carbohydrates, and break down very quickly). Raw fruits and veggies are suited to help maintain a healthy weight too, as long as you exercise adequately and eat protein rich foods, such as nuts and certain greens. Keep in mind that cooking, while it kills a lot of germs and makes stuff tasty, removes a lot of the natural nutrients foods contain, most notably vitamins.
Eating uncooked, fresh produce is pretty important to a good digestive system, too. The body tends to react badly when filled with too much processed and cooked stuff, and might even be pumping a great deal of white blood cells and generally treating food as bad, because your body's better adapted to eating a hardier, more natural diet. You can cut a lot of your digestive problems by making sure that a little over half of your diet is fresh, uncooked plant stuff. When you can afford it, lots of grocery stores offer a great selection of both fresh and frozen fruits and vegetables. If you want consistently cheap, read up on what's in season for the best prices - You'll get a good, fresh variety throughout the year, and save a few bucks because you aren't paying for stuff that's been carted over from the southern hemisphere. I don't follow the seasonal stuff, but I've got a good variety I enjoy if you want some ideas.
When you start a new diet, be sure to take it slowly. When I started my diet I was little too excited about it, and nearly hopped on it cold turkey. Trust me, if you're looking to stop digestive issues, that's the wrong thing to do. Make the change slowly, or you might end up getting sick. Take things one step at a time, and you'll eventually get used to eating natural foods.
In your current situation, you can make the best call for what your situation requires. Some easy changes you can make that can help is taking vitamins. They're relatively cheap, so you might be able to convince someone to get you a bottle, and it's hard to overdose on them. The daily values are kind of bogus, because they were based around the idea of "how little can we take until we get sick". You can, and probably should, take more than the daily values suggest through your food and other sources. Heck, most vitamins don't really have an unhealthy upper limit, besides Iron and animal-based vitamin A. A complete multi-vitamin can help with stuff beyond digestion, so they're not a bad thing to take anyways. Vitamins are really undervalued in the States, despite evidence that they're really important for a long, healthy life. In my case, I was experiencing a lot of depression and energy loss until I started taking them, especially the Vitamin B's (All of them). Now I feel great. Placebo? Maybe. I don't particularly care as long as I'm not depressed.
Also, perhaps the most important for a good digestion is drinking water. A lot of water. Bottled, tap, rain, it doesn't matter. Water helps with a lot of things, particularly digestion and blood flow, and hardly anyone drinks even close to what they need. Try drinking at least liter when you wake up in the morning, and then one or two more as the day goes on. You should at least be drinking two liters a day, even if you're sitting around doing nothing. Once you start getting active, you'll need to drink even more. Avoid drinking until you feel ill, though, because that's bad.
Since we're on the topic of water, avoid sodas, coffee, and maybe even tea. They can cause ulcers and other digestive unpleasantries. You're relatively young, so you may be sensitive to them.
I hope that helps.