You should study something that you are interested in.
You should take something that you really want to study and learn about.
ーYou should study something that you are interested in.
「興味のあることを学べばいいよ。」
study something that are you are interested in で「あなたの興味のあること/関心のあることを学ぶ」
ーYou should take something that you really want to study and learn about.
「あなたが本当に勉強して学びたいもの（授業）を取れば良いよ。」
something you really want to study and learn about で「あなたが本当に勉強して学びたいこと」
ご参考まで！
カナダ 
