Welcome, Rose. You'll get a lot of support here so don't hesitate to join in on the various threads on the boards.
Whether you want to make small changes or big changes all at once will depend on your personality and whether or not you really have "hit bottom." Personally, I tried small changes and sucked at it. When I finally hit my bottom, I decided to make a huge change, which has worked for me. The important thing is to experiment to find what works for you and DON'T give up. Remember, you're setting an example for your 5-year-old daughter who will follow in your dietary and lifestyle foot steps.
I'll echo Moonkissed: suck it up and buy the larger sizes you need to feel comfortable. Lay down the money for a couple sets of workout clothes--including workout bras and a pair of kicks with good support--so you can have one clean while the other is in the laundry.
Put some time into healthy menu planning when you're putting together your shopping list. The first two or three weeks may take extra time but once you identify the foods you need regularly, like salad greens, etc., the new shopping pattern will become just as routine as your current pattern of buying unhealthy, short-cut foods.
MsMatic is right, buy whole foods and drink barrels of water. If you can't afford fresh vegetables, go for frozen vegetables to avoid added salt and preservatives. I have a schedule of eating every three hours starting at 6 a.m. with my final meal of the day being 6 p.m. I stick to this schedule religiously. This helped me eliminate my habit of constantly grazing. When I feel the impulse to eat, I know my next meal or snack is only three hours away. No night time snacking.
Keep a food journal to track your caloric intake and get a good, firm grasp on how many calories are in the foods you eat. Also log your exercise. I, personally, find the site MyFitnessPal to be most helpful in this process. They have a mobile app, too, if you've got a smart phone. That way, you can journal on the run. SparkPeople also offers free tools, recipes, and tons of educational material.
Since I'm bulimic, I personally advise against using food as a reward so recommend you avoid a weekly "cheat meal." Personally, I've sought out lower calorie alternatives for preferred foods or have given them up altogether at this point of my weight loss journey. I reward myself with perfume, workout clothes, or a piece of jewelry. I've given myself a new hairstyle and bought name brand shampoo. These are simple things that I typically deny myself because of cost but really, if you see a $24 large, loaded pizza as a reward you'd spend money on for a cheat--save yourself the calories and get a cute pair of earrings instead.
Finally, ask for the support you need. Sit down with your boyfriend who loves to eat out and let him know you need to change your eating habits to lose weight and improve your health. Let him know your plan for food shopping and healthy meal prep at home. If he wants to eat out, he should do it at lunch time when he's at work and you won't be impacted by his choice of restaurant. Ask him to please support you in this lifestyle change. Maybe he'd like to learn to cook and help out in the kitchen? (In the end, all the money you save on eating out might finance a nice family vacation down the road!)
Don't leave your daughter out of the loop, either. Let her know that Mom needs to change the things she's been doing and you need her help to keep you active and help you exercise and make healthy meals. Spend quality time with her by teacher her how to wash and prepare fresh vegetables for salads. You'll be teaching her new, healthy habits that will last her a lifetime and she'll be so excited to spend time in the kitchen with Mom, she won't even care that she's peeling carrots instead of making cookies. When it comes time to do housework, put on some lively music and have a dance party with your daughter while you sort and fold laundry, dust and vacuum. Go for walks together at the park. Play tag. Whatever you can think of for active play that gets both of you moving.
When you plan to workout in a way that doesn't suit a 5-year-old's abilities, ask your boyfriend to watch her and give yourself the time you need to kick butt in the gym, swim laps, or go jogging. Anytime you need motivation, look at that beautiful little girl of yours and ask yourself if you want her grow up believing it is better to live in denial, wearing clothes that are too tight and dig deep, painful red lines into her skin just because she doesn't want to acknowledge that she's made some bad dietary choices. Wouldn't it be so much better to start making good choices for yourself while you teach her how to live comfortably in her own skin?
You've taken the first step. You can do this for yourself and your daughter. We're all pulling for you! It will take work. It won't be easy, but you've got this.