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! :)

you must say "Watashi NO ringo wa doko desu ka?" watashi no ringo = Me-of-apple = "My apple"