What to know to make money as freelance web developer?

I am new in software developement. and i don't know wich field to choose for specialization . there are so many fields in the world of web developement today. so many frameworks, so many langages, so many tools. And i know i cannot master all of them. However, I would like to be an excellent web developer, and be able to create, mobile application, and scallable and secured web applications. I am also passioned of mobile games development. According to you, what is the best choice for me. I am 28 years old and i would like to create my StartUp and work online.