If you want to get in great shape, for instance, then you will have to discipline yourself and sacrifice your favorite foods, put more time (perhaps a lot more time) into your workouts, and make lifestyle changes.
If you want to achieve career success, you are going to have to sacrifice your time, energy, and work ethic. When most people are going out and partying, you will have to be working hard, living pragmatic and disciplined, and giving yourself an advantage.
If you want to get a new college degree, you may have to sacrifice your current job and make time for your studies. You will also have to sacrifice the comfort of the life you are in to one of challenging studying and assignments. Everything you want to achieve in life is on the other side of discipline, sacrifice, and change. It's up to you if you want to grab it.
Sometimes the greatest discipline will be patience. Most people give up, but if you could stay suspended in time, and wait patiently while working hard, you can achieve your goals and greatest dreams. You just have to decide if it's worth it.
With the things you want most, it usually is. With your health and fitness goals, and the shape you want to be in, it always is. Discipline is the key to achievement.