Something has to go here first. That’s a rule, right? This is that thing. The first thing.
First things first.
Maybe you’re not familiar with that expression. Here’s a place to look up idioms like that: The Free Dictionary by Farlex. And here’s a link for the idiom in question. It’s not an ideal site for language learners, but that seventh tab, “Idioms,” is useful. You can also look up idioms via Merriam-Webster’s dictionary, though it doesn’t always include variants.
Maybe you’re like me. Maybe you thought there was an apostrophe in that phrase. It turns out there’s not. (Thanks Grammar Girl.) That just goes to show you, native speakers have blind spots. We make mistakes, too. Perhaps more so than language learners, actually. We’re usually less careful with idioms and expressions. Hence all the variants and eggcorns.
But you’ve got to get started. Even if it’s with an error.
*First THINGS first. (not … first THING’S first)