Basically calories in<calories out. You need to burn more than you eat. I find the best way to do this is to track what you eat and measure your portions. Even if it's just one day a week, it really, really helps. I've lost 60 lbs over 15 months doing this.
Exercise is important too, but it can really be as simple as going for a 20 minute walk 5 days a week at first. When I first started trying to lose weight, I couldn't get around the block without a rescue inhaler, so guess what? I walked halfway. Then I built up from there. Every little bit of exercise helped.
If your schedule fluctuates, then on a day where you do have time, go to the grocery store, buy yourself some proteins, fruits and veggies, and pack them up into meal portions. Hey presto, convenient lunches.
Here are my favorite convenient meals:
1. Cook up a couple of pounds of ground lean meat or meat substitute sometime when you have 30 minutes to spare. Freeze in 4-oz portions. When you're hungry, zap a portion in the microwave with 1 cup of some marinara/spaghetti sauce from a jar (like Classico or whatever) and a big handful of fresh spinach until the spinach is soft and everything's nice and hot (you can easily do this in a frying pan too). Eat alone or with 2 oz (dry weight) of a boiled whole-grain pasta or some steamed/baked potatoes.
2. Cook up wholegrain pancakes and freeze, separated by layers of waxed paper. Have one with 2 tbsp peanutbutter, some plain greek yogurt, a tablespoon or two of sweetener, and as much chopped fruit as you like
3. Take a great big handful of fresh spinach and cook it in a frying pan for a few minutes until wilted. Add a couple of eggs and some low-fat cheese and scramble.
4. Get one of those pre-cooked rotisserie chickens and tear all the meat off. Freeze in 4-oz portions. For a filling salad, thaw one portion of meat and add it to two big handfuls of salad greens, 1/2 an avocado, 1 orange/mango/handful of strawberries (or any fruit you like), and maybe a handful of broccoli slaw or shredded carrot. Add 2 tbsp vinaigrette dressing (I like Annie's personally).
These are just some ideas. I'm gluten intolerant, so my suggestions come from that standpoint. You could also try packing healthy sandwiches, pitas, etc. if you can eat gluten.