Say you start out with TOMATOES as a topic.
Ask WHO is it about: Tomato gardeners? People eating tomatoes? Is it about the tomatoes themselves?
Ask WHEN it happened: When did people start eating tomatoes? Are you thinking about modern tomatoes, or the history of tomatoes? Do you want to research the cultivation of tomatoes, or what happens when you cook tomatoes?
Ask WHERE it happened: Where did tomatoes come from? Where can you grow tomatoes?
Ask WHY it happened: Why did people start growing tomatoes? Why are most tomatoes red?
Ask HOW it happened: How did tomatoes become so popular in home gardens? How do you grow tomatoes?
Ask WHAT happened: What is the story of tomatoes? What is it about tomatoes that you really want to know more about? (This may be part of the answers to some of the other questions!)
You may not need to answer all six questions in your project, but it's a way to NARROW DOWN your topic.
So instead of picking TOMATOES as your project, now you can look at the answers and be more specific. There are all sorts of focused topics now, from choosing to research how to grow tomatoes in a home garden to finding out more about the early history of tomatoes. Pick what's interesting to you and run with it.
The best advice is: don't bite off more than you can chew!