В чем разница между first off и first up ?Пожалуйста, приведите примеры.

First off is usually used when saying your first argument when you have many arguments eg: "first off, you are wrong. Secondly, you are very wrong, third, you are really really wrong."). Can also be used with other listed things (other than arguments).

First up is used when introducing a list of things, topics, people (eg: "First up, we have Carol with a song. Then we will have Jaxon with a guitar solo.")

First off sounds a bit weird to me though. I think you cover all bases if you just stick with "first". Plain and simple.