Why begin with your diet?
1. Muscles aren't Everything
Alright, lets say you're the person that eats whatever you want and goes to the gym (also probably whenever you want) in order to build fitness. You probably even see some progress. Well, if you're not eating well while your muscles are getting enough fuel to get bigger and stronger (bigger probably because you're just now filling out your natural frame. Stronger because your CNS was so inefficient before) then your organs are probably having some difficulty keeping up. This is because every day you work out your organs have to deal with the breakdown of your body as well and aren't getting all the proper support they need. You can't keep this up for long before your body rejects you from making any more progress.
2. Muscle is Rebuilt in the Kitchen
Going to the gym does not necessarily make you stronger. For example, when fresh to working out and when after working out one can no longer perform as well athletically. That's because the person's muscle fibers are all ripped up and need some time healing back together. The only way they can do that is if they are properly fueled. This comes from a good diet. Another note, if you tear your fibers up too bad don't expect them to heal up very quickly. So if you work out biceps every day a week, there's the reason you still have 13" Pythons. ;)
3. Healthier
Because everything you do will be effected by the support of a good diet whereas only certain operations are effected by the support of good exercise; most notably, your capacity to experience and internalize information related to fitness objectively.
4. Required
No matter where somebody start with their fitness pursuits, this is always something that they will be required to eventually do.
How to diet
Step 1: Organization
One can't expect to have very luxurious understanding and success when it comes to projects, including a diet plan, unless they're organized. Keep track of what you eat. Now don't go overboard and try to write down every calorie and every type of calorie if you don't want to. Just do something that works for you. maybe that's just taking a glance and what's on the label and trying to remember it. You must however, be reading nutrition facts. ;)
Step 2: Information
The best result will always come from the most informed decision. Before you try throwing together a random diet based on what people tell you and the media shows you it does well to read a tad; not talking about reading all the latest gimmicks and what _exact_ food may be something to avoid, but rather the physiology and science behind what you are eating. Here's a inquisition method you could use to progressively learn relevant information:
1. What does protein do? What about carbs? And now fat?
2. What are all the types of fats? Trans? Saturated? Unsaturated?
3. Complex carbs vs. simple carbs?
4. What do amino acids do? Enzymes?
5. How long does it take for certain foods to digest?
6. What are ways digestion might be slowed down or sped up?
7. What are all these micronutrients and vitamins for?
8. Can I overdose on different vitamins?
You better have your own questions by here. :P
Step 3: Motivation
You probably won't like doing things you don't want to, so a little motivation can help. If you want you can keep track of your measurements, weight, exercise stats, etc. Other things like how fast you can solve a Sudoku (lame) puzzle are effected by health, too, keep in mind. Learn some neat quotes, whatever. Just keep yourself interested! (I do this by learning new things in the field (: )
By the way. You can use those same three steps for doing anything.
You're welcome (:
