Your song is the foundation of the whole routine, so it is worth choosing with the dancing in mind — not just the lyrics. A track with a clear, steady beat is far easier to move to than one that drifts, and a tempo that is neither too slow (which exposes every pause) nor too fast (which rushes you) is ideal for most couples.
Length matters more than people think. Three to four minutes of continuous dancing is a long time on a dance floor with everyone watching. Many couples shorten the song, or plan for the routine to build and then invite the wedding party or guests to join partway through — a lovely, low-pressure option.
Pick something that means something to you. The most memorable first dances are not the most technically complex; they are the ones where the couple clearly loves the song and looks like they are enjoying it. If you have two contenders, dance a little to each at home and notice which one makes you want to move.
Once you have a shortlist, share it with your coach early. The right choreography for a slow ballad is very different from an upbeat number, and knowing the song from the start means every lesson is building the actual routine.