What's pulling you?
If you really want to change, you have to decide that you want it, but more importantly, why you want it - knowing what drives you and what will keep you motivated is absolutely critical to achieving your dreams.
Most people won't (and don't) take the time to figure it out, these same people will get only a few weeks into their program before they lose motivation and they will be destined to fail because they never took the time to examine why they wanted to change in the first place and more importantly, what they were willing to do to achieve that change.
Having a clearly defined goal gives you a destination to aim for, but that isn't enough on its own. You also need to have a powerful driving force - an understanding of why you want to accomplish that goal - which will create a magnet that will pull you towards that destination. You will undoubtedly encounter obstacles over the course of your journey - everybody does. However, if you don't know exactly where it is you're going and why you want to go there, you'll easily be pulled off track and the smallest obstacle will cause you to give up. But if you know exactly where you're going, how you want to feel, and you have a powerful reason why - you can get through any obstacle, because the magnet will be strong enough to pull you towards your goal, making it easier not to give up until you get there.