It can be hard to know what to write about and if you're like me, coming up with ideas is the hardest part of writing. Writing prompts are a great place to start. Here's some places to find good ones.
Instagram or Facebook accounts that regularly post writing prompts (I follow @writing.prompt.s and @writing.prompts.re)
Writing prompt books: My roommate has a very cool one. It's like a desk calendar with 3 sections. One for the type of writing (article, short story, flash fiction, etc.), one with the subject, and one with a situation. Flip each one randomly and you could come up with something like "write an article about a shark who can talk." Super fun.
Fan-fiction: a lot of people are against this, but I think it's a great idea. Not at a professional level, but if you need a prompt to get your creativity flowing, there's nothing wrong with borrowing characters from someone else for a while.
Yourself: keep track of stories from your own life you could write as nonfiction or fictionalize. What are the stories you tell other people whenever you get a chance? Write those.
The real world: there are stories everywhere. Keep track of things you see throughout the day that could be stories and write them down. When you don't know what to write about, consult the list.
Look at the world a different way: this is my favorite because I love playing with POV. So I might write a story from the POV of a mailbox or a shoe or an animal.
Write about something weird: Is there something about the world you think is odd that other people don't seem to think is? Write about that.
Write about something the world would be weird without: what would happen if Saturdays were cancelled? Or if squirrels didn't exist?
Ask your friends to give you a place, subject, and situation. Write a story about that.
Another tip for writing prompts is to set a timer. I find when I don't have a time limit, I overthink the prompt and try to come up with the best story idea in the history of story ideas. Give yourself 30 minutes or less to write the story. If you have to think fast, you will. You can always go back and change it later. Some of the best writing comes from when you stop thinking about it.
P.S. If you just thought "oh, that won't work for me" try it anyway. This can work for anyone if you try.
You can also set a word or page count goal if that helps you finish. I like to write and see where the story takes me. Since I don't know what will happen yet, I don't know how long it needs to be. But if you need one, go for it.
Next, tell someone you're writing. They don't have to read it, but if someone else is expecting you to come up with something, you're more likely to.
Finally, let it suck. Writing prompts are usually not how you come up with your next novel idea. They exist for you to keep writing when you don't want to. They help defeat writers block. They unlock your creativity so you're able to come up with what your next project is. Plus, they might inspire your next project.
But for now, write it fast and write it bad. If you tell yourself it can be bad, you'll stop thinking. And remember what I said before? That's when some of the best writing happens.
Find a writing prompt and set a timer for half an hour. How much did you get done?
Start a list of events in your life that can be a story and start a list of events you see around you that can be a story (this doesn't have to be limited to people. Maybe you want to write a story about the two cats you see walking down your sidewalk each morning.)