How to test keywords for seo?

Hello, I'm trying to promote my own resource, most of the content on my site is training articles and Photoshop videos.

I create authoring content, but recently I noticed that my articles is stolen and published on other resources, while google first showing competitors sites using my keywords and then mine.

I think I’m just not composing the key phrases for the search correctly.

Tell me how can I check if I choose the right keywords for seo?