first of all; in the first place

First off, I'd like you to come with me to a department store sale.

First of all, the level of the ocean could rise, because ocean water expands as it grows warmer.

Firstly, let's check the attendance.

If you want to criticise other people, first you must improve your own methods.

Bigfoot is not a pet. For one thing, they don't like being called 'bigfoot'.