First, I studied A, and then I studied B, and lastly I studied C.
"First, I studied A, and then I studied B, and lastly I studied C."
「最初にAを勉強して、次にBを勉強して、最後にCを勉強した」
* first: まず、第一に
* firstly / secondly / lastly: 最初に、次に、最後に
* studied: 勉強した
ご参考になれば幸いです。