Don't be afraid to try multiple keyword combinations! It may take a few attempts before you find which combination works best in your chosen database. No article is going to cover every single aspect of your topic. It's your job to find articles relating to different pieces of your topic, then you can stitch them together with your own critical thinking.
When coming up with keywords, remember to keep them short and simple. Unlike Google, library databases do not work well with full sentences. It's best to avoid words like in, to, of and the. Consider replacing them with AND (see the Linking Keywords tab). When you're looking for peer-reviewed sources you should keep in mind that you need to use the same type of language as professionals. For example, in a peer-reviewed paper the author would likely use the term juveniles, not kids.
Let's say you're writing a paper on diversity in education. That is a really big topic. You need to focus your topic (narrow it down) before you begin searching in the databases.
What type or area of diversity are you interested in researching? This could be racial, socio-economic, cultural, religious, etc.
If you're searching in an education database, then all of the journals are related to education. That means education isn't going to be a great keyword and you need to be more specific. It could mean elementary schools, high schools, curriculum, classrooms, etc. Perhaps you would like to discuss inclusive pedagogy or afterschool activities. The databases don't know that's what you're looking for unless you tell them.
Thinking about these questions before you begin searching will help you form a list of potential keywords (and combinations to try). it may also help you to visualize your topic with a concept map.