First of all, you have to know if you're after a research article, or a review article. Once you know that, most databases and indexes are set up to allow you to do choose a topic and find articles on that topic. All you have to do is pick some words that describe the topic of the articles you want to find, and the database looks for articles on those topics.
The words that you choose to describe your topic are called "search terms." The database will try to find your search terms in the most important fields of each article. That is, if you run a search on "cows," the database will look for articles that have the word "cows" in the article title, the author, the journal title, or the abstract (summary) of the article. Sometimes this is referred to as "text word" searching, because the database is just looking the text of your search term to appear anywhere important.
Once you've chosen good search terms, just type them into the text box marked "subject," "topic," or "keywords." Some databases only have one text box to use. Type in your search terms and click the "go" or "search" button.
|Pub Med||Cancer, Estrogen, Mammogram|
|Web of Science||Oyster, Parasite|