Many people go through writers block and struggle with the concept of what they will write about. Everyone writes differently and there is not set way of making it work. Some write without the worry of what the story will turn out to be. Others want to envision what their story will be about.
You don't have to wait for an epiphany to burst into your mind to begin writing. I like to put an outline together of what I'm going to write about. Is that outline written in stone? No. Do I need to worry about it as I'm writing? No.
To me the outline is a map or guideline, if you will. Each point may begin to change as I write. Some may be removed completely, but what the outline does for me is start me off writing. And let's face it, how to start writing is usually your most difficult issue.
Should you do it my way? No. The point is that you develop a way to get yourself to write. If you write that first novel and your agent ask you to write a sequel or new novel. You are now faced with a deadline, and the pressure of knowing that you have to write something. There is nothing like pressure to give you writers block. If you have a system, this will not be an issue for you. Your system could be which days you will write and for how long. It could be where you look for ideas, or an outline, whatever it takes to get your mind thinking and your fingers typing.
Never allow fear, or anything else for that matter, stop you from writing. Come up with a system that will help you along, and always write.