When you say/write simple phrase, just the direct indicitave, "this (item)" this car, this house, this girl, this boy, this toy, then use the declining этот.

When you are saying something with "is" in English, "what is this?" "this is a(n)/my/his/her ..." then use non-declining form, это. "Это моя собака." "Что это такой?" etc.