Once you know your main research topic, you will start putting together a search strategy. This tells the database what you want to search. Consider using Boolean Operators like AND/OR/NOT, and using synonyms. For example, looking for research about therapy dogs effect on long term patients? Some researchers use the term "dog," or maybe "canine," or maybe a specific breed like Golden Retrievers. You want to make sure you get ALL the relevant results possible for your research.
An example search strategy could be: Therapy dog OR therapy canine AND feelings OR attitude OR emotions OR emotional state AND long term patients OR long term care OR extended hospital stay
Remember that databases search terms not meanings. If you search with a "NOT dogs" everything that has the four letters DOGS will be excluded, whether it is an author's name, journal abbreviation, or found in the title/abstract.
Sometimes you will need to explore a topic more fully, and will need to conduct a more comprehensive search. This is a big task, but creating a table like this can help you begin:
Creating a simple table like the one above, can be a really helpful guide to keeping your thoughts organized.
Combining that all together, the above search strategy looks like this:
Therapy dogs OR Therapy canine OR animal therapy OR Animal assisted therapy [MeSH] AND feelings OR attitude OR emotions OR emotional state OR mood OR Emotions [MeSH] AND long term patients OR long term care OR extended hospital stay OR Length of stay [MeSH]