If one is lying and one is telling the truth what do you ask?

You ask one of them "If I ask the other person, what answer will they give?" and then you do the opposite.

If you have asked the liar, he will tell you a lie about what the truth-teller one would tell you.

If you have asked the truth-teller, he will tell you the lie which the liar would tell you.

Let us suppose that you want to pick from a good door (door A) and a bad door (door B)

If your question is "If I ask the other person what door should I use. what door would they say?"

The truth-teller will know that the liar would say B, and will say this.

The liar would know that the truth-teller would say A, and will say B.

So, you need to base your answer on what they don't say - which in this case is Door A.