I write science fiction, mostly, though I'll write any story that comes to mind. I do have novels and short stories I'm working on, but what I really want to do is screenplays. For that to be a viable career, I'll have to move to Los Angeles.
As for where I get my ideas, the short answer is: everywhere. I watch the world around me, and pick up on ideas. I'll overhear conversations, listen to music, watch people go about their lives, sit back and daydream, and ask, "What if...?"
What if that guy over there was a secret agent? What if the world really was created by aliens? What if I was an alien and didn't know it?
Most of all, I read. I study stories and poetry to see how it's done, and I study the advice and methods of other writers. If you don't read fiction, you can't write it well. Most of my ideas get discarded to the ether, but I sometimes come across ideas that seem to have some weight to them. So, I sit down and write.
There once was a boy. Okay, what if? There once was a boy who was very sick. What if? There once was a boy who was very sick, until he wasn't. Overnight, he was made whole, in the midst of a dream.
From there, it's a matter of asking "what if" and "what next." You just get the words on the page, then go back and groom them into a story. It's that simple, and that hard.