Once you have defined the topic, the next step is to brainstorm for all of the possible words which describe the topic. These will be your keywords. An easy way to do this is to identify the different concepts in the topic and then list synonymous terms under each concept. For example:
voter turnout presidential elections voting electoral college popular vote candidates voters electorate
Notice that not all of the synonymous terms are true synonyms. What we are looking for are words that might show up in the information we need which refer to the same concepts. At this point, all descriptive words that come to mind are acceptable. As the search is refined some terms may be removed from the list and others added. Commonly used, undescriptive words such as an, the, in, out, off, etc. are not useful because most databases will not search for them. These words are called stop words.