If your living space is constantly messy and depressing, clean it up. Put on a fresh coat of paint. Vacuum the carpet. Clean off your desk. Wash the windows and let the sun shine in.
Beliefs also come from past experiences.
That is why it is so important to experience success on a regular basis. Set small, achievable goals, and then reach them. When you start to experience success, then you start to believe in success, and that leads to more and bigger successes. Beliefs come from knowledge. The more you learn, the more things you'll know to be possible.