Start by reading the articles referenced in other answers I've given. Then do some brainstorming. Get a paper and pen and write in the center "importance of water." Circle it. Now close your eyes and breathe deeply for a second to get centered.
What does that phrase remind you of? Draw lines out from your center circle and write single words or short phrases that come up when you think of the term "importance of water." You might write at the end of one line, "drinking." At the end of another, "keeping plants alive" and so on.
If drinking reminds you of other things, like health or blood flow or something else, draw a circle around "drinking" and add more lines out from it, putting the new words at the end of each line.
Then go back to the very center again and do the same with the next word that "importance of water" reminds you of. You'll end up with something that looks like a big spider web.
Now, look at the entire web. Do you like the way it's organized? The web helps you organize your paper. You can use it for an outline that flows naturally from the way you think. Each main word can be the title of a section, and all the words are radiating out from it can be the information that goes into that section.