first things first
The most important task gets priority, as in I very much wanted to see that movie, but first things first—the paper's due tomorrow. This adage was first recorded in 1545.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.