Consider the search volume for your chosen topic. Find out how many people are searching for that topic by running a web search for, say, "quick weight loss." The quotes will make your search specific, so that the only results will be sites that mention the exact phrase "quick weight loss." Your search engine should list the number of search results that turn up for your query. Try to find a topic that turns up fewer than 5,000 results.
The problem with affiliate marketing, like many other home business options, are the so-called gurus and get-rich-quick programs that suggest affiliate marketing can be done fast and with little effort. Odds are you've read claims of affiliate marketing programs that say you can make hundreds of thousands of dollars a month doing almost nothing ("Three clicks to rich!"). Or, they suggest you can set up your affiliate site, and then forget it, except to check your bank deposits.