Pushups are a great way to tone up your entire body, because they require the use of so many different muscle groups. Because the standard pushup position works out the abs, legs, chest, shoulders, and arms, they build a lot of muscle in different regions of the body in a short amount of time. Just holding the position, without performing any reps, is a great exercise for strengthening your core.
Because pushups are a portable exercise, i.e. they do not require the use of weights, they can easily be performed anytime, anywhere. This allows you to do your workout over the course of an entire day instead of cramming it into a brief trip to the gym- although compressing your pushups into a timed workout can give you some amazing results as well. Doing pushups all day long will keep your metabolism fired up all day long. This is just like splitting up your cardio workout into morning and evening workouts, instead of doing it all at one time. When you work out, your metabolism speeds up for a period of time, then it gradually slows back down. By dividing up your workout into smaller workouts, spaced out evenly throughout the day, you can sustain the metabolism boost from your workout for a longer time.
The number of calories burned by pushups is difficult to calculate because of weight differences in individuals and the speed that you perform your reps/sets; and, daily caloric needs vary from person to person. For these reasons, I cannot recommend specific dietary changes that you should make in order to build muscle and lose fat, but I will provide some general suggestions. It is important to make sure that you get sufficient protein, water, vitamins, minerals, and carbohydrates. Check your protein sources to make sure that they are "clean", i.e. not a lot of fat- so that you do not gain fat with your muscle- and try to avoid sugars. Hydration is very important for a multitude of reasons; muscle function, organ function, tissue function, joint function, etc. Beyond the basics of eating healthy and drinking plenty of fluids, the rest is up to the workouts.
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