Vたいです expresses the speaker's wish or desire to do something.
The subject of the sentence is the first person わたし in the statement. In a question, the second person is used as the subject(e.g., あなたはVたいですか). Note that Vたいです is not used with a third person subject.
Vたい changes its ending in the same way as i-adjectives do; therefore, its negative equivalent is Vたくない.
わたしは 友だちと りょこうしたいです。
I want to travel with my friends.
あなたは 外交官に なりたいですか。
Do you want to become a diplomat?
→いいえ、わたしは 外交官には なりたくないです。
No, I don't want to be a diplomat.