。
I'm on the swimming team.
Literal
I [topic-は] swimming-club [in-に] am-entered.
部に入っている ('be in a club') uses ~ている to express the resulting state of having joined. 水泳部 = swimming club (水泳 'swimming' + 部 'club/section'). Polite form ~ています.
。
I'm on the swimming team.