"How do you get your ideas?" is the second most question I get asked. First thing, and this is going to sound like a total douchebag response, I don't know how to NOT have ideas. If you're hardwired to create they just come.
When people try to "give" their ideas, they always start with "I have this great idea--" and I jump in and say "Me too. And I'll never have time to write all of them." And this is kinda true, my list grows almost daily. They aren't all GOOD ideas mind, and most of them are just fragments. The big thing I want to say on this, is it's very rare to have a full complete movie drop into my head. It happens, but not often. Most people have idea fragments, like tiny little story atoms bouncing around in your head.
The 1st thing YOU have to do: Write down the tiny idea atoms. It may be an idea for a character, a location, a story beat, a situation, a scene. You may be having ideas like this and ignoring them because you don't know what to do with them or they don't have more to them. START WRITING THESE DOWN. I personally use Evernote because I can access it on any device at any time. When things pop in, I throw them onto my list. If I don't I will definitely forget them. And here's the thing-- If you have a list of little atoms, and you look at it, one of two things can happen. Either these end up serving as your own little writing prompts that you can expand on, or the little atoms start getting attracted to each other and begin to form molecules.
For me, once mini idea atoms cling to each other I start to get a picture. A character idea clings to a setting, clings to a plot. If this doesn't happen for you, look at your mini ideas as questions. Read them and ask "Why is this happening? What situation begat this?
Like I said, these can become prompts to explore. If you're really not coming up with even the little bits, there's countless resources for writer's prompts out there. I think the Writer Emergency Pack is a great one for beginners, but there's countless free prompts online.
The easiest way to see if your idea has legs as a story is to rework it into a premise or logline. There's a lot of ways to do that, lot's of people who think they should be formatted a certain way, but I find that most often the simpler the better. I like to do it like this:
Lead character + setting + key conflict = premise. Example: "A detective that hunts androids in a dystopian future struggles with his humanity as his targets become more human than he is."
If none of this is working for you, there's always another way... one of my go-to’s: use a model. Anyone who has ever wanted to write has watched a movie and thought "This would be so much better if they did X." So do X. Think of your favorite film, keep the plot, but then flip everything else. This is WAY more common than you'd ever think. SCREAM, which I maintain is one of the most teachable horror films ever, takes all the beats of a Victorian drawing room mystery and places it in the world of a slasher film. ALIEN is basically a haunted house with a monster movie, but the house happens to be in space.
Hope this helps. Being creative is organic and conceptual so it's hard to make it tangible and teachable, but I bet most people have more ideas than they realize, it's just getting into the habit of documenting them, examining them, asking questions about them, to make them full.