たい is an auxiliary verb that is used to express things that you would like to do (want). To use たい, it needs to be added to the same conjugation base form of a verb that ます would be attached to.

• 今夜（こんや）はピザが食（た）べたい。
  Tonight, I want to eat pizza.
• プロの歌手（かしゅ）と歌（うた）いたい。
  I want to sing with a professional singer.

When using たい, the thing that is ‘wanted’ can either be marked with を, or が. The difference in nuance is that something marked with が is considered to be wanted more. たい is conjugated in the same way as い-Adjectives, meaning that the negative, past, or past-negative forms may also be used.

• 虫（むし）は食（た）べたくない。
  I don't want to eat insects.
• 警察官（けいさつかん）になりたかった。
  I wanted to become a police officer.
• 昔（むかし）は先生（せんせい）になりたくなかった。
  In the past, I did not want to become a teacher.

たい is only used for expressing things that ‘you’ want, or repeating information/asking questions about what other people want. It may not be used to make guesses about what other people ‘may’ want, unless asking them directly.

• 何（なに）が食（た）べたい?
  What do you want to eat?
• オレンジジュースを飲（の）みたかったの？
  Did you want to drink orange juice?