I am a 22 year old guy with elbow problems. Eight years before, I fell down from the stairs and my elbow fractured. The doctor could not help me, so I am physically disabled. I cannot straighten my left arm to more than 145 degrees. My body type is ectomorph and my friends make fun of my skinny body. I want to gain weight. Should I join a gym or would I be skinny forever?
The most important thing about gaining muscle mass with a skinny physique ('ectomorph body type') is the diet. Your diet has to be both high caloric and high protein, use an online calorie calculator to roughly work out how much you should consume per day (the bodybuilding website usually has a few). When fixing your diet, try to obviously only consume healthy products, I would recommend things like: eggs, milk, porridge, wholegrain rice, bananas, cottage cheese and avocados, which tend to have either a high caloric or protein count.
The best fitness programme I would recommend to someone starting off, would be the stronglifts 5x5 -http://stronglifts.com/5x5/-. Which basically involves going to the gym 3 times per week, and carrying out 3 compound exercises -squats, bench press, barbell row or squats, overhead press and deadlift -.
Now concerning your elbow, I would recommend seeing a doctor or more ideally a physician to see if your arm can physically take the strain of these exercises, if not then you could do the following for each exercise:
Squats - I would recommend the mid back version, as your arms are only needed to balance the barbell, so shouldn't be too much of a problem, though I would check just in case.
Deadlift - You could use a strap on the shoulder with the bad arm to help take the strain or get a friend to help you.
Bench-press/ Overhead press - these exercise may not be suitable if you can't take any weight on your bad arm however if you can just try and extend as far as possible and get someone to support you in case your arm gives way.
Barbell row - again may not be suitable if you can't take any weight on your arm. Regarding your arm not fully extending, start the exercise with your arms bent (not straight) and then just carry through the range of motion from there.
Hope that helped, best of luck.