Believe in yourself because no one knows what it takes to be you, you are the only one that knows your dreams and aspirations, therefore you are the one to show both to yourself and the world what you are capable of. You will realize that when you start something new majority of the people may be skeptical about its success, very few people will believe in you, but this is the time when self belief should carry you through and push you.
Self belief will help you do things confidently and where you meet obstacles it will help you persevere. Self belief can be natural where one has grown up in an encouraging environment however it may not come naturally sometimes and that is when you need to build it. It can be built through doing different things like doing something new for example visiting a new place by yourself, learning a new skill or sport. This somehow reminds you that you are capable of different things.
It also expands your brain and gives you belief that you can do many other things. Self belief is mostly cultivated through doing therefore if you are grooming yourself for things like sports competitions you can do that through practicing often and you can also use affirmations to remind yourself that you are capable of anything.