First of all; in the first place — used to state the first or most important point.
まず第一に、健康が何より大切だ。
First of all, health is more important than anything.
まず第一に確認すべきは、パスポートの有効期限です。
The first thing to check is the passport's expiration date.
会議ではまず第一に目的をはっきりさせよう。
At the meeting, let's clarify the purpose first.