put one's best foot forward

To try to act as an ideal version of oneself, typically to try to impress others. You really need to put your best foot forward in the interview if you want to get this job.

Try for the best possible impression, make a good start, as in Come on, let's put our best foot forward for this interview. The allusion in this idiom is unclear, though it may concern marching. One theory is that best foot means "the right foot," the left being regarded as unlucky. [Late 1500s]

To try to make the best possible impression. There is something inherently puzzling about this expression, which dates from the sixteenth century. What exactly is one’s “best foot,” and why should it signify putting on a good show? Shakespeare made it the better foot (in Titus Andronicus and King John), and Sir Thomas Overby wrote, in 1613 (Characters: A footeman), “His legs are not matches, for he is still setting the best foot forward.” One writer suggests that “best foot” always meant “right foot,” the left being considered unlucky. Whatever the explanation, the metaphor is still current.