First and foremost I'd like to stress the importance of safety for people ready to begin a training program. As with any form of exercise, it is greatly advised to consult with a doctor in order to ensure your safety while putting your body under new (and tougher) conditions.
With that said, congratulations on having this much drive to begin a training regimen, and let's begin.
Step 1: Knowledge Is Power
You know your body better than anyone else
Every body is different
I would recommend strongly against taking any one source of information as truth (including this Instructable!). What works for some other people may not work for you. Self-knowledge is VERY valuable when you are working out. Much of the time that building muscle and strength is used figuring what works for you. As you advance in your training, you will learn more and more about your body, its strengths and weaknesses and how it will respond to different exercises, weights, rest periods, and other variables.
I would recommend having (or acquiring) some knowledge on nutrition, anatomy, and resistance training in general. These can be found in workout manuals (big books which look like telephone books). These are good for reference and getting new ideas for different workouts and exercises to put in your routine.
Step 2: The Basics
As a simplified example: If a person eats 2000 calories in a day and burns 1800, they will gain weight.
Conversely, if a person consumes fewer calories than burned, they will lose weight. (If the INs equal the OUTs, the weight will be steady).
There are (in the real world) many more factors in the real world, however, including metabolism and genetics but those are really more of asides.
The important thing that you need to know is that in order to get bigger, you must have more INs than OUTs. If you are at a stable weight now, you will need to eat more (good) food. If you don't work out now and are at a stable weight, you will need to eat even more (because you will be burning more calories now).
So lets look at some of the things to have in our diet...
Step 3: Nutrition
In your diet, you will need more carbohydrates, protein and calories. Carbohydrates give the body energy, proteins enable muscle growth after a workout, and more calories in general means (more energy as calorie is a unit of heat) your body will gain weight (and if you do it properly, it will be mostly muscle).
Don't be tempted to think that this means gain weight in any way (through fast food and other fatty foods). If anything, you must be even more conscious about your diet than before you started working out. Your increased carb intake should come from breads, pasta, rice etc (preferably whole grain, whole wheat, and wild rice where possible as these carry other benefits as well (fiber, vitamins etc.) )
Proteins should come from (if possible) from lean animals in one form or another. If not, though beans and nuts also have a lot of protein.
Fish - Very high protein to fat ratio
Chicken - Similarly high
Lean meats - Where possible eat the leanest meats to avoid putting on fat
Egg - High in protein (if your family has a history of high cholesterol avoid the yolk)
Milk - Quintessential. If you don't love it, you should learn to love it. High protein and almost no fat.
Yogurt - Similar to milk, though slightly more fatty
Cheese - VERY fatty although there are leaner cheeses with a lot of protein and little fat
Soy Beans - Probably the best thing for vegans to eat, enormous amounts of protein
Beans - Likewise
Nuts - In moderation, because like cheese they are VERY fatty
Your calorie intake should increase as you increase consumption of these good proteins and carbs. Don't ignore the fruits and veggies either, they have a lot of vitamins and fiber as well.
I'll expand from this overview in the next update.
Step 4: Body Weight - Push Ups
These are to be done with hands shoulder width apart, and back straight with your core (abs and lower back muscles) tight. Lower yourself in a controlled manner until your chest just makes contact with the floor and then press through the chest, triceps and shoulders back to the first position (as demonstrated in the first five repetitions).
To diversify this staple exercise, I have included a recommended workout (one that I do as well). The first set consists of standard push ups. In the second set, called Triangle Push ups, the grip is much closer (forming a triangle with the hands). This variation puts more emphasis on the triceps. The third and final set consists of wide grip push ups to put emphasis back on the pectorals. The form of the standard and wide grip push up are identical, but when doing the triangle push ups, you may need to shift your hands down (so that you are pressing closer to your stomach) and separate your feet in order to keep balanced.
I do these three in succession with no rest between. I would recommend doing a set (five of so) of each to get the hang of them then doing successive sets. Start with few reps in each of the three sets, then as you begin getting stronger in the days and weeks, increase your reps. Doing about four sets of the three exercises should be enough, but like I said every body is different. Remember to keep your back straight throughout the workouts (form is more important than number of reps!).
A note: If you feel like doing push ups in this way puts too much stress on your shoulders, stop the repetition when your chest is about 5 cm (2 in) above the ground.
Step 5: Body Weight - Dips
Note: If you cannot yet complete a dip, try a modified dip which is the same form, but you put your feet up on something approximately the same height as your hands. Then in a "sitting" position preform the dips. This can be done outside of a gym if you have a bench and a chair: put your palms on the bench (fingers outward) and your feet on the chair. (Video coming soon)
Step 6: Body Weight - Overgrip Pull Ups
This is a very good exercise for the latissimus dorsi (or back muscles), biceps and forearms. Different grip positions work different parts on the back. This exercise at the (wide) grip I use focuses on the upper back.
Note: "Kipping" or kicking up your legs to help thrust you up is not advisable (at this stage, at least) because it makes the exercise less concentrated to the latissimus dorsi. A more advisable solution if you have trouble completing repetitions is to have a spotter (someone helping you) hold your feet (with your knees bent 90 degrees). This way, you keep the focus on your back but can keep your form through the desired number of reps (remember, form is more important than number of reps or amount of weight!)
Step 7: Body Weight - Under Grip Pull Ups
This exercise is meant to be done in addition to the over grip pull ups.
I usually start with over grips for a few (about 3) sets and then do the same number of under grips. The same spotting method from the last exercise is very valuable in this exercise as well.
Step 8: Next Steps...
After your body has had some chance to get used to the new diet and workout schedule (around two weeks should be sufficient), it will be time to move onto additional resistance lifting techniques (like training with weights and machines). Although this is not necessary, lifting with body weight exercises is one of the safest ways to resistance training and it is possible to get substantial results through using only your body as resistance.
Remember; form of reps is much more important than number of reps or weight! Doing the correct form from the beginning will give you more results faster.
For all of those who are curious about lifting with free weights, my next instructable will be covering some free weight lifting techniques. Stay tuned and good luck with your workouts!