MY HEALTH AT FIRST

Fitness, for me, is about meeting your goals for your body and your life. What does that mean for you? Do you want to be able to run a marathon (or a mile, for that matter)? Lose weight? Gain muscle mass? Jump higher? These goals are very different from one another. The very fit man who can do this:
is probably not also going to be able to do this:
(And I can do neither.) But I am in pretty good shape for what I am, and so I'd like to share with you some very basic things that I wish I knew about diet and exercise when I was younger. Why?
I had always been pretty thin, but when I was seventeen, I got very sick and lost a lot of weight. By time the illness was over, I weighed an extremely slight 128 pounds, despite being my full-grown height of 5'9" (176 cm). I didn't like not feeling strong, sure, but more than that, I didn't like how limited I felt in my body. If I knew what I know now, what would I tell that kid?
Some simple exercises a few times a week and some small changes in your eating habits can really help you build muscle. If I were starting to lift weights at a gym for the first time, here are the basics of what I would want to know, in the language I learned it. (Probably not correct physiological terminology, but you can always check an anatomical chart.)
1.) If you want to build muscles, you need to exercise the muscle groups you want to build. For your upper body, that means your chest (pecs), your back (lats and traps), shoulders/neck, triceps, biceps, and core (abs). For your lower body, that means your butt (glutes), your thighs (quads/hamstrings), and your calves. There are a whole host of free-weight and machine exercises, as well as some body-weight (i.e., equipment-free) exercises that are easily google-able for each muscle group. Whatever it is you're going to do, warm-up first! This could be as simple as five minutes of walking briskly on a treadmill. Getting your blood flowing and your muscles warm is a great idea before doing any weight-lifting.
2.) Choose a reasonable weight! I've overdone it many, many times, and you probably will, too, as you learn what your body can handle. When I first started, I could only benchpress 75 lbs. That's the truth. The weight I would choose, when I'm starting off, is whatever weight I can -- with excellent form -- do about 8-12 repetitions of the exercise before my muscles are too fatigued to do another repetition. If I can do more than 12, the weight is too light, and if I can do fewer than 8 (or my form suffers), the weight is too heavy. Doing 3 or 4 sets of each exercise (the number of repetitions you can do should decrease by 1 or 2 from set-to-set) will give you a good workout.
3.) Do two or three different exercises for each muscle group. Don't just do a benchpress if you're working your chest; do pushups or cable crosses or something else to exercise that muscle in a different way. If you're just starting out, I would start at the low end (2 exercises of 3 sets each = 6 sets total) for each muscle group. If you're going to work out 3x a week, you can do three different muscle groups each time you work out. Work your three muscle groups for the day in descending size order, from largest to smallest. (If you work your biceps before your pecs, you will find out the hard way that this is a recipe for pain!) This way, by the end of each week, you will have worked out all of your major muscle groups!
4.) Cool down and stretch! This is supremely important for maintaining flexibility and avoiding injury. You'll be tired at the end of your workout, but do it anyway. All of this takes about 90 minutes at a gym three times a week. You can always switch things up on yourself, but this was a great starting point for me that I wish I just knew when I was starting out, instead of learning slowly over the course of many years. And while you do this, make sure you stay hydrated! I typically burn through about two liters of water during a workout like this; discover what's right for your body, and err on the side of overhydration.
But that's just the exercise part. Is there more to it than that? Yes, but it isn't that hard. The two other components to get your body into shape, other than exercise?
Rest! That's right, after working your body this hard, it needs to recover. That means you need a good night's sleep, and it really helps with recovery -- especially when you're starting out -- to not lift weights two days in a row. The other major part of how to build muscle?
Eating appropriately. If you want to gain muscle, that means both more calories and the right kinds of calories. I like the website DailyBurn to track my diet and exercise, because they have a huge food database and show you the breakdown of where your calories come from. I'm not going to tell you what you can or cannot eat, but if your goal is to build muscle, you should be getting a lot more protein than the USRDA says. My general rule of thumb is that for every pound (kilogram) you weigh, you should be eating 0.8 grams per pound (1.75 grams per kilogram) of protein. So if you want to gain muscle and you weigh 175 pounds (79 kg), you should strive to eat about 140 grams of protein per day.
Not all at once; it should be spaced out fairly evenly (maybe 30 grams at a time) throughout the day (in four or more small/moderate meals). It's a fairly significant lifestyle overhaul for most people, but these were changes that I made that were relatively easy for me to make. Most of all, I'm really happy that I'm a lot more conscious of what I eat and drink overall, and that I feel good with what my body can do. And that part, feeling good with your body, is the greatest thing I've gained from being fit, and, however you choose to go about it, whether you choose to follow Abbie's or Isis' or Sci's or anyone else's (or your own) plan, I wish you the best in meeting your fitness goals.
The above? Those were the changes I -- mostly -- made to my life during my 20s. What I'm telling you isn't a scientific study, this is just my own personal experience and what I would recommend, based on my experience, to a novice who wanted to gain some muscle and didn't know where to start. Because it's very tempting to start with an ad like this (with compelling before-and-after photos):