Read On Writing by Stephen King. His “method” is not for everyone (there’s a reason he’s a multimillionaire and we’re not), but you will definitely be inspired to just sit down and write something, which is the only possible first step. You just gotta let some words come out, see what they are, and go from there. All the technique, planning, and honing the craft can come after that first step, or in later projects.
The big takeaway from that book does apply to everyone:
“If you want to be a writer, you must do two things above all others: read a lot and write a lot.”
If you don’t know how people come up with ideas, you probably just haven’t given yourself a fair shot at it. Everyone has ideas. Sit down, open up Word, and see what happens.
If you still have a lot of trouble, try a writing prompt.
http://www.writersdigest.com/prompts - These are pretty good.
https://www.reddit.com/r/WritingPrompts/ - I don’t think these are as good because too many of them try to railroad you into a story the prompter has already thought of.