。
I'm not very good at tennis.
Literal
I [topic-は] not-much tennis [subject-が] skilled is-not.
上手 is the standard na-adjective for 'skilled, good at.' Like other ability/preference adjectives, it takes が for the thing one is good at — テニスが上手 = 'good at tennis.' あまり~ない softens the negation. Note the polite form 上手ではない vs the colloquial 上手じゃない.