(1) I want to sing. わたし　は　うたいたい 　　　　　　　　　うたう（verb）　＋　たい（auxiliary verb）＝うたいたい (2) She wants to sing. 　　かのじょ　は　うたいたい　( I think it isn't uncorrect 　　　　　　　　　→かのじょ　は　うたいたがっている 　　　　　　　　　→かのじょ　は　うたいたい　と　おもっている（I think this is natural expression) (3) I want to go to school →わたしは　がっこうに　いきたい

１．Describing your own desires: ます-stem Verb+ たい I want to sing ＝ 歌いたい I want to go to school = 学校に行きたい＝学校に通いたい (I want to attend school) ２．Describing the desires of others: Same conjugation as above in #1, but instead of たい put たがっている (It seems like...) He/She wants to sing =歌いたがっている You didn't ask, but another related expression is telling somebody you want them to do something. I want him to sing = 歌ってほしい