I would love to help you! I deeply enjoy writing but the problem is, I can't! As a writer I understand that not everybody finds passion in the same things as I do, but if you put your heart into it, you will do great! Although I can't write the entire thing, I can give you some tips on writing this story:
-Try to think of something you would want to find. Make it sort of a mystery! Have you always wanted to find a... talking dog? Then maybe make this secret cave the home of a talking dog! You can develop a story from anything that you want!
-Don't rush it! If there is a deadline try to make a schedule that works for you and your family, but, if you are like me and stuff pops up all over the place, work on it whenever there is time! You don't have to make it perfect, just write whatever comes to your head.
-If you still can't think of anything, talk to a teacher. They will be happier if you turn to them instead of turn in nothing because you couldn't figure it out and didn't want to ask for help.
I hope these helped you! If you have any other questions feel free to comment on this and I will try to answer it!