As for how long it will take... it really depends on how hard you work and how dedicated you are to getting in shape.
It sounds to me like you have some "skinny fat" and you might benefit from building a bit of muscle.
For food.. you definitely need to eat more often to get your metabolism revved.
I would eat more protein for breakfast... try adding in some eggs or low fat cheese or some kind of lean meat. Also make sure the cereal is low in sugar and high in grains and fiber.
I would also eat a snack between breakfast and lunch... some sort of protein and complex carb.
For lunch, make sure the sandwhich is on whole wheat bread and stay away from highly processed meats and things like mayonase.
For dinner, stay away from greasy foods and try to stick to complex carbs, veggies, and lean meats. I know this is hard as you are eating what your parents cook but you could always talk to them about wanting to eat healthier or just try to make the best choices with what you have.
I would have a snack after dinner in the form of lean protein and veggies.
Here is something that i recently wrote for another person... maybe it will help?
Perhaps this well help....
In order to lose fat and gain muscle we need a combination of lean proteins, complex carbs, healthy fats, fruits and veggies, and of course water.
Lean proteins: Protein is vital to building muslce, which aids in fat loss, increased metabolism, and injury prevention. For best results i try to aim for 1 gram of protein per pound of bodyweight each day. Examples of lean proteins include: skinless chicken breast, egg whites, lean red meats (venison is my favorite), fish (the fatty fish like salmon are good too as they provide healthy fats), some slices of pork, soy, and low fat and fat free cheese/cottage cheese.
Complex Carbs: Complex carbs are usually those that have undergone little or no processing. White flour and sugar are both simple carbs. Complex carbs are best as they are broken down into glucose more slowly than simple carbs, providing a steady amount of energy instead of a sudden spurt, followed by a sudden drop. Examples of complex carbs include: oatmeal, sweet potatoes, whole grains, legumes, brown/wild rice, whole grain bread/pasta/couscous, quinoa, and high fiber whole gain cereals.
Healthy Fats: In recent years many have associated fatty food with body weight and obesity and as a result many drastically cut fat out of their diets. The truth about fat is that it is a vital part of every healthy diet and serves a wide range of purposes. Healthy fats help with brain function, cell function, heart function, lung function, eye function, digestion, and immune system function. Examples of healthy fats include: flax, fish, avacados, nuts, coconut oil, and olive oil.
Fruits and veggies: We all know that fruits and veggies are an important part of a healthy diet. I try to watch my portion sizes when eating fruits higher in fructose and limit these to my morning meals when my metabolism is at its peak. However, fruits like cucumbers, zucchini, tomatoes, and peppers can be eaten throughout the day as they are low in sugar. When choosing veggies I usually go for things like asparagus, spinach, brocolli, and green beans. When choosing fruits I try to stick to thinks like apples, bananas (these are a great pre-workout food) berries, grapefruit, cucumbers, peppers, zucchini, and tomatoes.
Now that brings us to macro breakdowns. Throughout this forum you will often read about macro breakdowns.... a concept that i found very confusing when i first started. Macro breakdowns breaks protein, carbs (this includes complex carbs, fruits, and veggies), and fat down into ratios. The macro breakdown of 33/33/33 means that a diet is 33% protein, 33% carbs and 33% fat. The same goes for 40/30/30... this reflects a diet that is 40% protein, 30% carb and 30% fat. A diet that is 40/35/25 is 40% protein, 35% carb, and 25% fat.
So how do you calculate your macros for a certain number of calories? Lets assume a person is eating 2000 calories a day and wants to eat a macro breakdown of 40/35/25. They would need to figure out how many grams of protein, carb, and fat to eat each day in order to get this ratio while eating 2000 calories.
So to figure out how many grams of protein to eat you would take 2000 x .4 (calories time percentage of calories that will come from protein)=800 So 800 of this persons calories will come from protein. Now, we know that here are 4 calories per gram of protein so we simply take 800 / 4 to get 200. So this person will be eating 200 grams of protein a day.
To figure carbs we do the same process: 2000x.35=700 and 700/4=175 so this person would be eating 175 grams of protein per day.
To figure fats you again do the same process: 2000x.25=500. Now fats are different than proteins and carbs as there are 9 calories in one gram of fat. So to figure grams on this example we would need to divide 500 by 9 to come up with 55.56 or simply 56 grams.
Next we want to make sure our math is accurate so we simple add 500+700+800 and we get 2000 so we know these macros are correct for a 2000 calorie diet with macros of 40/35/25 (remember the macros always must add up to 100%).
One last thing... what does your workout routine look like? Simply doing situps will not help because fat reduction is pretty much impossible. You can work the muscles like crazy (as sit ups will do), however if you do not lose the fat on top of the muscles your hardwork will be for nothing. Have you ever lifted weights before? What kind of exercise equipment do you have access too?