1. Eat Right
If you are over weight you should eat a diet high in protein, about .9-1.5g per weight of lean body mass in lbs. Moderate to low carbs, and lots of essential fatty acids. Keep the food groups in there but eat less. For those who are skinny and want to put on size just eat more keep up the protein and make sure you're lifting.
2. Train Hard
You can't get jacked if you aren't willing to put in the work for it. Sure you might be skinny and have a six pack but that doesn't mean squat that just means you have low body fat percentage (bf%). You're still a little man.
3. Train in Muscle Groups
Don't go to the gym and go from bench to squat and finish with some crunches, train one part of your body each day. Your body will focus its repair and blood flow to one area at a time. You've only got so much your body has to work with so make sure its focused in one area for maximum results. This also helps by allowing greater repair time for trained muscle groups while you work other the next day.
4. Remember What Lifting is...
A lot of people have their own idea of what working out is, some people seem to think its some sort of magical process that you just become stronger the more you go, keep in mind that what you're doing is tearing down your muscle fibers through strain and exhaustion and letting your body rebuild them larger each time you do it. Its all about adaptation and putting your body into a mode in which it believes it must develop these muscles to survive.
5. Use Full Range of Motion
This is self explanatory, use proper lifting technique while lifting, nice flowing movements and none of that short jerky stuff.
6. Don't Hurt Yourself
Another no brainer, make sure you don't over do it and only lift what you can, also this goes along with the movement. One mistake could permamently weaken an entire muscle group or worse.
Cardio is a must for a good physique and endurance. Its not only an amazing fat burner but is great for your health and likely to entend years to your life whe regularly done. Most people including me hate it but you've gotta stick with it and do the best you can. Cardio should be no less than 40-60 minutes and up to 2 hours of it a day for the average human being is great to. If you can't run for more than a minute, just walk at a brisk pace and add an incline, or bike, or swim. Find something that works for you, and learn your heart rate.
We live in a time where everyone wants everything immidiately. Expect this to take awhile, none of that 90 days or 300 hundred workout stuff works as advertised. You'll be in much better shape at the end but most of the people used for those were already hitting the gym for years before they even learned of the program and I'm sorry its not possible to get ridiculous results in a month even with anabolic steroids and 4 hours a day in the gym. You'll see results but not like those you see on television.
9. Know What Kind of Build You're Going for
Do you want to look like a massive body builder or lean and athletic, remember higher weights with low reps (1-7) builds bulk and power while lower weights with higher reps (8-15) builds leaner muscle and increases endurance.
10. Do it for Yourself
Everyone has their own reasons to get in shape but the best is to do it for yourself, you'll feel better, be healthier and have more confidence when you workout regularly. If you're just doing it to get women, you're more likely to let yourself go when you have one. Just make sure you know what you're doing when you're at the gym and have fun.