Something I find helpful for most of my more "involved" stories is outlining. There are all kinds of writers out there, and some probably prefer to just jump into a piece, which is fine too, but if you're not one of those, consider making an outline. This can be anything from just the "bare bones" of what you want to write, to a more complex, scene-by-scene skeleton of the entire plot.
I personally prefer a scene-by-scene outline, because if I don't know where I want a piece to go, I will often pause on it and might not get back to it. When I was in college, one helpful method of outlining I learned (which I still use today) was to bullet point your main ideas, and then place related ideas underneath. For example, your main idea could be "Set in a Subway" and then under it would be characters that are in the subway and what takes place there.
If you're more the scene-by-scene type, something I've found helpful is to type out exactly what you want to happen when. This does get tedious for longer, more complex pieces with many plotlines, but as I stated earlier, the choice is yours which outline type you use (if any).
Maybe you have no idea where you even want to begin. That's okay, too. I've been there many times and still get stuck quite a bit. But one pattern I've noticed is that giving up and simply saying "Oh well, there's nothing to write about" isn't usually productive.
A technique I use (which was partially stolen from here), is to create a character and give them a history, then decide what they want and why they can't get it or what they have to overcome to get there.
There! Wasn't that easy? You already have at least one character with a goal and a conflict that forms the plot of the story. Maybe a second character stands in their way also? The character's goal can be anything from wanting someone's love and acceptance, to seeking world domination. You can give your characters multiple goals to achieve throughout a story as well.
Possibly the best advice I've ever received from a writing website was that characters need to react to events rather than just being swept up by them (I don't recall what website, or I'd provide the link). This is something I struggle with as well in my own work; characters are unique people, and your story should show how they think and react rather than how you would think or react in a similar situation.
At the bottom of this page, you'll find some resources I use to outline my stories, such as links to free mind mapping software and sites that provide intriguing prompts and ideas. Enjoy! (All links open in the same window you're currently viewing this in).
Mind Mapping Software (Requires Free Account)
25 Types of Plot Outlines
