Once you've zoomed in on a subject...

"I'm upset over what happened to Princess Diana and want to write about what should be done about the paparazzi."

1) Describe your topic in a sentence:
There should be laws that prevent paparazzi from bothering famous people.

2) Identify the important or "key" words that describe your topic:

Laws  Paparazzi  Famous People

3) For each keyword, compile a list of related words and synonyms.

Legislation Journalists Celebrities
Photographers Movie Stars

This process can help you better define and focus the scope of your research project. Of course, you won't use ALL of these words in your keyword search; some are more appropriate than others, depending on your topic.