The First
First, Beginning, Outset
noun, の adjective
The most first thing is the first thing.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
最初は元気だったんですが、1時間後にはヘトヘトでした。
I was doing well at first, but I got exhausted an hour later.
中学校の最初のクラスは英語でした。
The first class at junior high school was English.
日本に来た最初の頃はトイレにビデが付いてるなんて知らなかったよ。
When I first came to Japan, I didn't know that the toilet had a bidet.