ほしい – when you want something
Use like suki/kirai: [something] が ほしい です
くるま が ほしい です = I want a car
なに が ほしい ですか？ = What do you want?
〜たい Form – when you want to do something
Stem of verb (polite form without ます) + たい
たべます (to eat) -> たべ (stem) + たい = たべたい (want to eat)
Use conjugation of i-Adjectives so just conjugate たい like any i-Adjective:
たべたい want to eat
たべたかった wanted to wat
たべたくない not want to eat
たべたくなかった did not want to eat