。
I'm not good at sports.
Literal
I [topic-は] sports [subject-が] weak-point is.
苦手 is a na-adjective meaning 'weak point, poor at, disliked.' Like 好き and 嫌い, it takes が to mark what one is bad at — because the 'thing one is bad at' is grammatically the subject of the predicate 苦手. The subtext of 苦手 often extends to 'dislike,' not just incompetence.