第一（だいいち） is an expression in Japanese which may be used either as a 副詞（ふくし） (adverb), or as a 名詞（めいし） (noun). As a noun, it will carry the meaning of ‘foremost’, or ‘most important’. As an adverb, it can mean ‘first’, or ‘above anything’.
To use 第一（だいいち） adverbially, pair it with に. As a noun, it will usually be followed by の (when connecting nouns), or by the 助動詞（じょどうし） (auxiliary verb) だ, when at the end of a sentence.

• 第一（だいいち）に彼（かれ）は何（なん）であんな時間（じかん）にここにいたんだ。
  First of all, why was he here at that time?
• これはここの第一（だいいち）のルールだ。
  This is the number one rule here.
• お金（かね）よりも健康（けんこう）でいることが第一（だいいち）だ。
  More than money, being healthy is the most important thing.

Occasionally, まず ‘first of all’ will be paired with 第一（だいいち）. This may seem redundant, but is like saying ‘the very first’ in English.

• まず第一（だいいち）にこの計画（けいかく）を担当（たんとう）する人（ひと）を決（き）めましょう。
  Firstly, let's pick the person that will be in charge of this plan.