I was in the harmonica club in high school.

Literal

I [topic-は] high-school era, harmonica-club [in-に] was-entered.

高校時代 ('high school era') — 時代 marks a period of time. ハーモニカ部 — ハーモニカ is an English loanword, 部 is 'club.' 入っていました (polite past ~ていた) expresses a past state of membership.