Think of your brain as being an onion, and every idea is a layer of that onion. When you write down one idea, it peels off a layer and exposes the one underneath. Then you can peel off that layer and get to the one under there, and so on, and so on, and so on. Eventually you empty out all your ideas, probably make a few new ones, and you have a big long list of ideas that you can now sort through and use. It's important to remember that if you don't peel off that first layer... you'll never find what's underneath. Often the good ideas will start 5 or 6 items down the list!
How to brainstorm
Your brain is a BIG onion! You'll have a lot of ideas! To brainstorm just make a LIST of those ideas. Write quickly! Think on the paper!!! (Note the word STORM - it should be intense!)
It is a good idea to practice Brainstorming. By starting small and getting used to it, you will become ready to tackle more complex topics.
Suggested topics for practice: