The First
First, Number One, Best
adverb, noun
The ordinal number prefix one refers to the first thing. This is the number one place you can get. In fact, it's simply the best.
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.
自分の安全を第一に考えて下さい。
Please think of your safety first.
休みたい第一の理由は時間が足りないからです。
The number one reason I want to take a break is that I don't have enough time.
お互いの第一印象はどうでしたか?
What was your first impression of each other?