Finding good keywords is essential. It would preferably be the words, terms, concepts that are often used in your subject area..
Sometimes you may need to use several related words, terms, concepts to find relevant material. You combine these words with OR, for example, agricultural waste OR biomass OR biofuel. This way either one or more of these words will be included in the material you find. Then, after some time you will maybe have to add some other words that will instead help to specify and focus your search. You do that by combining the search words with AND. If you select Advanced search, you will be assisted by the search form on the screen.
You can make other settings that limit the material you find before you begin your search. This includes:
You can also filter the search results afterwards.
In many databases you can combine previous searches with the Search History function: