Adverb
first up (not comparable)
(sequence, idiomatic) Firstly; before anything else.
Look on the bright side," said Simon. "If they need a human sacrifice, you can always offer me. I'm not sure the rest of you qualify anyway." Jace brightened. "It's always nice when someone volunteers to be the first up against the wall. Cassandra Clare
The cow that's first up gets the first of the dew. Scottish Proverb