I will let you know when I'd like to have my lessons when we have our next lesson because I don't know what my plans are yet.
ーI will let you know when I'd like to have my lessons when we have our next lesson because I don't know what my plans are yet.
「まだ予定が未定なので、次のレッスンのときにレッスンをいつ受けたいか知らせます。」
I will let you know で「お知らせします」
have my lessons で「レッスンを受ける」
I don't know what my plans are yet 「まだ予定がわからない」
ご参考まで!