Ok... for your situation, unless you're good at doing things cold turkey, read on...
Forget weight training (and definitely squats) for the moment. At 400lbs, your body weight is doing plenty for your legs and body as you move around.
First thing is changing what you eat. It's easy to say eat veggies, oatmeal, etc., but that's a difficult change for someone eating fast food most of the time. You have to start adjusting gradually.
Why do you eat fast food? Taste? No time to cook? Can't bring lunch?
Like hamburgers/cheesesteaks? Buy whole wheat bread, turkey/chicken burgers, and low-fat cheese with a little mayo.
Like fried chicken? Buy rotisserie chicken at the market, make mashed potatoes at home (with low-fat milk and smart balance butter).
Like sodas? Mix ginger ale with sparkling water or unsweetened tea with agave.
Follow those lines of slowly shifting out of fast food and slowly into homemade meals, by mimicking what you eat out with making it at home. It doesn't always save money, but saves calories and adds nutrition.
If you have to eat out, find places like Chipotle (chicken w/ brown rice, beans, lots of veggies), Pot Belly's (turkey or roast beef on whole grain), Kebab joints (grilled meats on rice, hold the butter). Go light on mayo/dressing/cheese.
Second thing is movement. Start slowly, with walking, no stairs yet. Do you have a treadmill or a park? Either way, start at a nice walking pace. Do 5 minutes first day, even if you could do 10. Add 30 seconds each day. It will add up faster than you'd think. Once you get up to 60 minutes at that pace, stay at 60 and up your pace (.1 on a treadmill) each week. If you don't have time for 60 minutes, at least max out at 30.
After a month or two, you can adjust your meals to be more healthy and add a little weight training to your routine.
Slow and steady is key to maintaining a major change in lifestyle both physically and mentally.