If you don't know the song you are trying to lead, . . cut it, . . .and do a different song. By "know", I mean:
a) Do you know the chord changes and how to play the chords? (capos are encouraged and helpful)
b) Do you have a good enough knowledge of the words to lead the song? (more to come on this)
c) Can you sing the song? (i.e. is it in your range) if not, you could transpose perhaps? Can't transpose? cut the song. . .do a different song.
d) There is an artful way to lead songs with words that you don't have memorized. Have you thought about how to do that? Takes practice, young grasshopper.
e) If the words, or concept of "karaoke" have been uttered. . . cut the song. Pick a different song.
We all make mistakes. . I've made more than most. But, these are some good "before we even get started" questions. Better to have less impressive, or easier congregational singing, than trying to "pull something off" that won't help folks sing together.