When you're saying something in Japanese, WA (は)marks the subject, and it can't be said more than once in a sentence. So the particle that you would need to use would be NO (の)which is used for possessives.
So what you would want to say is:
わたしのりんごはどこですか
Watashi NO ringo WA doko desu ka?
You can Also replace apple with anything and it should still work.
Hope I helped! :)