Summarize this article in one sentence.
That old saying "you are what you eat" exists for a reason: it's important that you eat the correct proportions of quality foods to get the minerals, vitamins, protein, and healthy fats you need for muscle development. In particular, be sure to eat plenty of protein, which your body uses to rebuild muscle you've broken down in workouts. Try to eat some high protein food about 20 minutes before and again directly after a strength-training workout. The amount of calories you should take in per day varies by age, gender, and lifestyle. Drinking water will help to reduce your sense of hunger and also allow you to exercise longer and better. Your body is approximately water and it must continuously be replaced to make up for what is lost through breathing, sweat, urination, and other bodily functions. Teenagers should try to drink 8 to 10 glasses, or 2 liters of water per day. Adults should have between 2.2 and 3 liters of water every day. In order to improve your overall fitness level, you're going to need to get active more often, even if you're busy. That means you need to design a fitness plan for yourself. Physical trainers recommend that individuals take four to five days a week to work out. If you're looking to build up muscle, then you might spend three to four days for strength training, one day on cardio, and two days of your week "active resting," which refers to doing something light like yoga or walking. In order to have good results, it is of the utmost important that you have a routine and that you stick with it. It is important to have cardiovascular workouts incorporated into your routine in order to help reduce fat since fat lays on top of and can hide muscle. Your goals must be concrete and reasonable or else you may give up. If you never exercise currently and you want rock-hard thighs, you will need to start with routines that are less strenuous (like incorporating smaller weights, shorter cardio workouts, and fewer reps) and work your way up. Some specialists suggest that in one year's time, beginners can gain up to 10% of their total body weight or 2 lbs of muscle per month with dedicated training.

Summary:
Eat right. Drink plenty of water. Design a regular fitness schedule. Have reasonable goals and a timeline in mind.