Journals, search engines, and indexing and abstracting services classify papers using keywords. Thus, an accurate list of keywords will ensure correct indexing and help showcase your research to interested groups.
This in turn will increase the chances of your paper being cited.
Read through your paper and list down the terms/phrases that are used repeatedly in the text.
Ensure that this list includes all your main key terms/phrases and a few additional key phrases.
Include common abbreviations of terms.
Now, refer to a common vocabulary/term list or indexing standard in your discipline and ensure that the terms you have used match those used in these resources.