！
I want an MP3 player!
Literal
MP3 player [subject-が] want!
A minimal, childish wish. ほしい ('want') is an i-adjective used for wanting objects (distinct from ~たい for wanting to do actions). The object wanted is marked with が. The sentence is the bare topic-comment pattern with no particles on the subject, plus an exclamation mark for the insistent tone. Very short sentences like this are common when the emotional force matters more than elaboration.