FANDOM. I'm not even kidding. I know a lot of places/people kind of look down on fanfic and fandom RPGs and stuff, but no joke it is a great way to get started creatively because you're building off of someone else's ideas, so you get a jump start. And just because it starts in fandom doesn't mean it has to stay there, either. Look at 50 Shades of Grey. That was a Twilight fanfiction that's now coming to theatres at some point! A lot of my plot ideas were generated as fanfic ideas. It's a good place to start if you feel super uncreative.
Also Zen's idea about writing prompts is really good. Sometimes I feel like I don't have any ideas for plots, but I have ideas for characters! So I'll make a character, find some writing prompts, and see what comes of it. Look for inspiration literally everywhere. Anything can be a story. If you're bored in class, think about your character being bored in class, and how to solve their boredom. Maybe there's graffiti on your desk. Maybe it could mean something.
Getting the ideas can be hard, and explaining the ideas are even harder, but once you have a plot in mind, the only way to get better at describing it is to practice. If you post a plot idea and it's unclear, people will ask questions that you will then answer, which will clarify things for them and also let you know what you can improve on in your descriptions.
Also, just because you don't think it's a good idea doesn't necessarily mean it's a bad idea. If you can write about it, it's viable. You can develop it more on your own and make it more player-friendly if you're so inclined, but don't immediately give up on something because it doesn't seem like a good idea at first. :)