question tag

Yes.
But there is no tag.

But what if i want to ask someone, then how it would form like

It's already in the form of a question.
Just ask
Why would someone have something on me?

Do you know what the idiom 'have something on someone' means?

Yes it's crush

Why would someone have something on me ?

No. You need to say
Why would someone have a crush on me?

Liton DasYes it's crush

No, it's criminal evidence, or something that could be used for blackmail.

Here is a an example of question tags.

You suspect that Janet has a crush on you. You suspect that your friend Sam knows this, too. You ask Sam, "Someone has a crush on me, don't they?"
Sam answers, "Yes, Janet does. But don't tell her that I told you so."

You then ask Janet, "You have a crush on me, don't you?"
Janet blushes. She doesn't know what to say.

Thanks your example has made my confusion clear, thanks a lot