Take a piece of paper. Write your idea in the middle of the paper. For example, if you're thinking about starting your own blog OR you already have one, you could write "My blog". Then write 4 new ideas about "My blog" around your main idea. Always add 4 new ideas around your ideas. HERE you can find more info.
Once again, write your idea on a piece of paper (as a topic). Then start attacking your idea. Write down negative things about it; what is wrong with it, why it's impossible and stupid. After this, start defending your idea by answering every single negative thing. This way you can test whether your idea is good.
Picture your idea as a tree trunk. (Once again, drawing this on a piece of paper will help you to visualize this properly.) Your idea might lead you to something (branches) and even further (leaves). What happens to your idea tree when fall and winter comes? Is it strong enough when the leaves start to fall. And what about if someone cuts off a brach? Do you think your tree is still good and something you want to have?