かみ
paper
kami
JMdict 1311530
紙(かみ)paperが欲(ほ)しいwant
I need some paper
が (subject) marks 紙(かみ) (paper) as what is wanted. 欲しい(ほしい) (want) is an i-adjective, not a verb — in Japanese you describe the desired thing as "wanted," so the wanted object takes が (subject) rather than the usual object particle.