That's going to depend on how much weight you want to lose, if you have any physical disabilities, and if you have other physical limitations such as hormone disorders.
Start by eating a healthier diet if you can. It's not accessible to everyone due to cost usually, but do what you can. Make efforts to go to the store once every 2-3 days and buy only what you know you'll cook in that time so it's as fresh as possible.
Stay hydrated. There's plenty of websites out there you can look at to determine how much you should be drinking based on biological gender, height and weight. Make sure you get at least that much if not a little more, you'll see at least some weight loss right away just from doing that as your body realizes it doesn't need to hold water like a camel in order to survive since you're giving it enough. Give it more if you're exercising, especially.
Exercise as best you can. I can't walk due to a physical disability, but I can do other forms of exercise such as biking and weight lifting. Find what you can do and do it, but start small and work your way up so you don't overwhelm yourself.
If possible see a doctor. Make sure you don't have any other physical disorders that would make a weight loss scenario difficult and get yourself medicated if you need to in order to help control it to make weight loss easier. Also make sure your plan for weight loss is healthy.
Do not starve yourself! In most cases if your body does not get enough calories yes, it will start pulling from your fat reserves but it will also go into a state where it stores more calories from everything you eat because it's trying to survive. Talking to your doctor or a nutritionist can help you get a good idea of how many calories you should be consuming in a given day, but it's usually around 1,500.
Don't be surprised or go overboard if your weight dips and then climbs again. Muscle is more dense than fat and thus weighs more; as you start gaining muscle mass due to the exercise your weight will increase a little after the initial loss. But this is also good because more muscle means more fat burning.
30 days isn't a lot of time for weight loss, it needs to be something sustained over a long period of time. But these are all good places to start and in 30 days you should at least see some improvement.