Protein
The most essential element of building lean tissue. Without it, you cannot grow. A good goal to go for is 1 gram of protein per pound of body weight. Most sources of protein have excellent amounts of useful amino acids. These molecules have to act with hormones to strengthen and expand muscle.
Calories
If you're consuming enough protein, the need for calories is usually met. The body needs more energy to repair, strengthen and grow new mass. Since proteins
have energy, the body can deplete available protein by using too much energy.
Health
Not all hormones are anabolic. Some are catabolic, destroying muscle. The body produces more catabolic hormones during sickness or stress. By trying to relax and plan ahead, you can help avoid stress.
Fats
Fat is essential to growth. Testosterone is formed from cholesterol, so if the body has a lack of cholesterol, less of this hormone is made. But don't pig out, since the body can't turn all cholesterol into testosterone. A diet that contains 30% of calories from fats is alright.
Exercise
Exercise is what makes muscle break down and rebuild, increasing strength and size. A minimum of about 3 hours a week of weightlifting is needed for growth.
That is, you'd have to break up the muscle groups into three days of training. One day of training is followed by one day of rest. The chest depends on the st strength of the tricep for bench presses, so working those two groups is a good idea. That's just an example; basically, you want to work a group that does the pushing one day, and groups that do pulling another day. While you work one muscle, such as the bicep, there are complimentary muscles that work at the same time, such as the forearm.
The maximum to work out is about 6 hours a week. It's possible to exercise three groups 3 days in a row and get two cycles a week in. You can also try working out twice a day for 3 days a week. This idea is the best for getting back lost muscle or building size quickly. By giving yourself a rest for 4-5 hours between the first and second sessions, it allows the body to begin repairing and building while you consume more protein and calories. Then when you go back to work out for the second time, the body has nutrients available to continue repairing and rebuilding. You'd want to use heavier weights during the first workout, and consume 30-40% of your total protein and calorie needs between sessions.
Self Esteem
This is important to your morale. Make realistic goals for yourself and stick to them. If you go to a gym, you may be embarassed by the progress of some individuals, but everyone has to start out some time. The way to get respect and advice is to not be a jerk. When you use a piece of equipment, do 4-5 sets, wipe the parts (if needed) and go to something else. Use weights you can handle, and don't make noise. It's ok to exhale or even grunt if it'll help you, but not on the first rep! And put the weights back where you got them.
Eventually, people will notice you have good form, and you aren't screaming while you bench 135lbs. You'll be asked for advice, and you can get help from others as well.
Almost forgot, the fastest way to annoy someone in the gym is to grab a machine without looking to see if someone is using it. If there's a towel or other belongings, don't throw it on the floor unless the person has been gone for a long time (5-10 mins). Some gyms have policies against taking up machines, but it's common sense. I've had situations where I'm walking to a piece of equipment from all the way across the room, and someone with less distance to walk, goes to it at the same time and ignores me. It's better to do something, no matter how insignificant, than to get upset and stop going. Suggest he/she works inbetween your sets (it's almost always a guy).
Motivation
This is probably the most important of all. You have to committ yourself to your goals and the necessities of getting there. You need to exercise on a routine schedule, and eat your goal of nutrients. There is always some flexibility, but if you can do it, then do it.
If you follow the recipe for success above, you will be satisfied by your progress, and it will become easier along the way as you feel better about yourself.