When you start programming your first web app you are doing something that you have never done before. It's a new challenge, a new opportunity, and in the case of software development, something so specific that it's probably never been done before.
It's normal to feel scared and unsure about yourself as you first start. But as you keep working on it, you will get the hang of it. And after that there isn't fear, there is confidence.
No one ever said they knew what they were doing when they start out in a new venture, or about navigating the turbulent throes of life! Most people pretend to know what they're doing, and that's okay.
Confidence is key in life, find things that you're good at, and make it happen.
This is a normal part of the process of learning a new technology, and in fact, it's a critical part of the process.
It takes 10,000 hours to become a master portrait painter, and it will take just as much to become a skilled professional. Believe in yourself, you can do it.