The quality of your search results greatly depends on using the adequate search terms.
To explore your research domain and define your subject, you could use general search engines, browse through (scientific) web directories, utilize (online) reference works or go through scientific and/or review journals.
It is important to deduct a clearly formulated problem definition from the assigned subject. What is your exact assignment? What (definitely not) to do? How far can you / do you need to go when searching for information on your subject? When is your deadline?...
From that clearly defined problem definition, keywords can be derived, i.e. a limited number of key concepts that are directly relevant to your assignment.
By complementing keywords of your problem definition with translations, synonyms, singular or plural form, narrower and/or broader and/or related terms, you obtain a good set of adequate search terms.
How to combine search terms?
Technique 1: Boolean operators (AND, OR, NOT)
AND: Used to search only for information that contains both search terms (intersection).
OR: Used to search for information that contains at least one of both search terms (union).
NOT: Used to search for information that contains the first but not the second search term (set difference).
Technique 2: Truncation symbols (wildcards)
* (asterisk): used to replace one or multiple characters. Generally, this symbol can only be used after the word stem (f.e. Herodot* for Herodotos and Herodotus).
? (question mark): used to replace one character. This symbol can be used after a word and in some databases also in the middle of a word (f.e. wom?n for woman and women).
Technique 3: Exact word combination (“string search”)
If you wish to find only search results that contain the entered search terms in that exact order, enclose your search terms in "double quotation marks" (f.e. "Martin Luther King").
Technique 4: Filters and search field specification
Filters can be used to narrow the search results of a search query. On online databases/catalogs, these are usually offered at the Advanced search function.
In advanced search, often it is also possible to search by several fields at the same time.