If you get too many articles in your search results, you need to narrow your focus. Add another term to your keyword. If you can't think of one, look at the keywords used by the database or find the keywords in an article seems to be close to what you want. In an article, keywords are usually above or below the abstract.
If you get too few articles in your search results, you are probably using too many keywords in your search. Try removing the least important keyword and doing the search again. Check the spelling of your keyword(s) and look for alternative spellings (e.g., peroxisome was once called microbody).