To select keywords for a search, start by writing your topic out as a sentence or question. For example:
"How are mothers and sisters depicted in Perrault’s and Grimms’ tales?"
There are several aspects to this question. First, extract key terms from the topic question (bold). Then list synonymous or related terms for each - you have a list of keywords to use!
Use this online keyword generator to create your lists in preparation to search online databases.