How can I build my back muscles ?

Here are some great

Do repetitions of sit ups. Lower yourself slowly down supported by your hands on the side of a bed or any support( your back should face the bed with hands behind you), with your legs straightened while you chest out. Pull yourself up at a faster rate than lowering yourself down. Do this for a month you'll realize some changes at your lower and back muscles.

This poses a critical question! although there are several different exercises to build the back musels. One, must not just do anything one might hear, without first researching, the full extent of the possible negitive results. I have been building my body for several years. My son is probuble one of the more renound body builders in his weight divasion at this time. I will tell you as I told him. First do your research. Then figure out what you want to accomplish with your back. Then, preferably see a professional. Preferably a specialist in this area. Finally, you need to understand, anything you do with your back, is going to be painful, until you reach your goal. At the atainment, of your goal there will be maintainence in order to keep what you have accompolished. I hope I have helped. This is a decision, not to be taken lightly! If you have farther questions please, by all means dont hesatate to get in touch. This is why I am here! 

The back is a notoriously challenging muscle group to build because it's not actually a single muscle that you can train with one exercise, but rather a chain of interlinked muscle groups that require you perform several different exercises that work the back through different angles and planes of motion.

In general, you can train the back effectively by incorporating two basic movements into your workout routine. The first are pull-ups, which are one of the best bodyweight exercises for the back, period. If you can't perform a full repetition on your own, use a pull-up station at the gym with some assist, or have someone spot you and help you perform the movement until you start becoming stronger and can do them on your own. Varying your hand/arm position and grip (underhand versus overhand) will allow you to emphasize the back a little differently each workout.

Don't get frustrated if you can't do many pull-ups to begin with. They are very challenging and with practice, you will get better.

You also want to incorporate a pulling exercise that has you moving the resistance perpendicular to your torso as well. Good exercises for this include seated cable rows, machine rows, or bent-over dumbbell or barbell rows. Start with a light weight to begin with until you get the hang of it. It's not about moving heavy weight, it's about getting a full range of motion and feeling a strong contraction at the top of the movement as you pull your shoulder blades together.  You can add weight as you become stronger and have a good feel for form and execution.

Perform these exercises early in your workout, before you work your biceps, since the biceps are required as a supporting muscle group to properly execute the exercises.

Both of these exercises assume that you are in good health, without any chronic back problems or issues with shoulder joints (which take on some stress, especially with pull-ups.) If you have a back condition, it's best to consult with your physician or a physical therapist before trying these exercises. A trainer with rehab background can also help you strengthen your back while reducing the risk of injury.

Best of luck!