I used to have the same problem, last year in seventh grade. Over the summer, I realized that you can't just simply COME UP with the plots, trying to sit there and think. Yes, eventually you will think of something, by just sitting there, but it will take an awful long time. Instead, think about your setting, your characters, think about other books you've read (not saying to copy off them... that's illegal. Just think about them) and also think about your characters personality. After a while, something will click. And as soon as that happens, don't wait to write it down. Take it from my experience. The first time it clicked, I sat around, not waiting to write it down, and a couple of minutes later my brain like... almost exploded with thoughts about the book, and plots, and plots for others books... etc. Write it down as soon as you think of it. Or, maybe you've already thought of it, but you didn't realize it. Do you remember any dreams you've had? If you're anything like me (which you kind of are.. the whole writer thing and all that) then you probably dream a lot. Think about your dreams, and how maybe they someone would be able to connect to your characters and your book.
There is a great book called Seize the Story. It is very very very helpful. Find a copy in your public library or you can buy it online. Trust me, I used to suck at writing completely, but that has helped my imagination immensely
A good plot usually has a few of these components; Well developed characters main goal of characters conflict (Ie something that keeps them from reaching their goal, or somthing like a war, a lost friend...) climax (the highest point of conflict where resolution or defeat is determined.) aftermath conclusion
Sometimes it helps to plot your plot like a movie script, thinking in terms of dialog and movie scenes. Put whatever you think is interesting, exciting, mysterious, fun, or sad into these scenes and tie them all together with in your setting. Include lots of twist and turns, unexpected events and outcomes that no one saw coming. And have fun with it. Sometimes you just need to learn as you go! :-)