Think about your first idea as a place to start -- not as your final question or statement
Check your assignment for types and number of sources required
Explore! Learn about your idea, read more than you have to, discover aspects that lead you closer to a question or statement that fits the size/scope of your project
Test your developing idea by searching for resources that meet your assignment guidelines
Narrow or broaden your focus depending upon your results
Repeat until you get the quality and quantity of resources that don't "drown" you, but help you to cover your particular focus thoroughly