There are really only two sure fire ways to make it in Hollywood, nepotism or being born into it.
Hollywood isn't a hundred yard dash, it's a marathon.
You wouldn't buy a train ticket if you didn't know your destination, never start writing a screenplay until you know where you are going.
There is no such thing as screenwriting, only screen rewriting.
If you have one idea for a movie, it's probably not very good. If you have 10, maybe one might be good. But if you have 100, then one of them will be fantastic.