If you think it is hard to gain weight, then you must just be doing something wrong. Everyone has the ability to gain weight, it just requires the right combination of a weight gain diet and exercise. But if you are looking to get a muscular body like all those body builders it is going to require a lot more effort as well as discipline and mental focus.
Weight gain not only requires a change in diet but also adequate training to ensure that you increase in weight is healthy. If you are skinny, then consider training 3 times a week for half an hour or more. I would minimise cardio and focus on muscle building exercises as cardio burns off the calories that you need to gain weight.
Working out can be boring though, so to help you stick at your routine it is always best to have a workout partner. Your workout partner is there to encourage you to keep going, to push out those extra reps, and they keep you at your program - and statistics prove that after 21 days of consistently doing something you create a habit. So after 21 days you have a healthy habit of working out.
Muscles only grow when they are at breaking point. You will need to stress your muscles so much that they actually tear - that is when you feel the burn. this allows your muscles to grow back stronger than they were before. And they will become bigger than they were before as well.