Following a fitness course can be challenging, especially if it's not targetted specifically towards you, the individual.
A generic fitness course is generally written for the average person. If you have specific goals in mind or are overweight, underweight or suffering from a physical injury or disability you will need to consult a personal trainer to put together a tailored fitness course for you.
A qualified personal trainer can consult with you to create a tailored fitness course that will meet your goals and expectations whilst also taking your current level of fitness into consideration.
You should also talk to a nutritionist to ensure you are balancing your calorie intake with the amount of exercise you are doing.
Remember that it really helps if you have someone to train with. Ideally that person should be of a similar level of ability to you. Having someone to train with and someone to motivate you and that you can motivate will dramatically increase your chances of completing your fitness course.