A good gym instructor can make all the difference in the world for a person going to the gym. It can sometimes be difficult finding a good gym instructor but you should try and sign up for a number of different classes that teach different ways to exercise to help you find the right type of instructor. Now the type of instructor you have can vary depending on what you are trying to do. Remember that if you fall in to a niche group or want to lose weight because of some event, then you might be better off going to a personal trainer who specializes in working with that niche group.
A good trainer should have defined terms as to what they are able to do and how much they are able to help you. An example is women who are trying to lose weight after a pregnancy. There are specific exercises and things that you need to do in that situation and for that you need to go to a gym instructor who specializes with the people in that group.
Aside from knowing their boundaries, a good gym instructor takes ownership of what you do and achieve while in the gym. If you are not disciplined then they will have to make sure that you form some sort of control and routine in your life. Gym instructors often have to whip people in to shape by telling them exactly what to do and then hold them accountable for not being able to achieve it. Quite often people will think about an extra cheat meal or having a few extra drinks at the bar but these things can ruin the entire routine that they have and self control is needed. You can read more about them online and on health blogs.