Believe in Yourself

Success doesn't come to those who only hope for it, or those who "think" they might be able to accomplish it. You must believe that you can do it, and that you are in control of whether or not you achieve what you desire. Whether you think you can or cannot, you are right. Achievement starts in your mind.
Whatever your goals or desires are, stop your mind from thinking about all the obstacles in the way, and how hard it will be to get there. Think only about getting there, and tell yourself that you will get there. Even if you do not really believe it at first, tell yourself anyway as if you do, and you will find your mindset slowly changing until you do in fact believe you can.
Remember to surround yourself with others that are a positive influence on you. Don't share your goals or desires with those who will only be negative and hurt your belief in yourself. People that think they can't are great at telling you that you can't either. Share with and be around those who also believe in you, and know you are capable of achieving what you desire.
Whatever the mind can concieve and believe, the mind can achieve. Don't limit yourself to only what you think is possible now. Whatever your mind can think up, you can achieve it if you believe that you can. It will take effort and dedication, but you can do it if you focus your mind on believing that you can and doing what it takes to make it happen.
If you find yourself not believing that you can achieve something, try to figure out why you think that. Is it that you have never accomplished something like that before? People accomplish new things every day. Is it that you don't have the skills neccessary? Find out how to get them, or find other people who have them who you can partner with. Whatever your "excuse" there is always a way around it if you look for it.
Practice shifting your mind to belief in yourself and what you can accomplish. Throw out any negative thoughts, and change them to positive ones. Believe you can accomplish anything, and it will become a self-fulfilling prophecy.