There's plenty of info on the web about choir leading, but the majority of it is geared toward the typical, sheet music-reading form of choir. You have to learn it to become prepared to educate a song. Review the melody till you understand it forward and backward, till it becomes a part of you. Listen to the melody and sing along with it. Exercise performing the soprano aspect and soon the entire part can be sung by you precisely. Or you could need to speed up the melody or slow down it to get a distinct feel.
Good quality ideas from a choir director pal of mine: sometimes you may want to simplify a song. In case you are planning to have a restricted quantity of testing time, you might leave out an elaborate passing or stay with only one vamp (saying refrain) if the melody may have 2 or 50 song 3 of these. On the flipside, if you're planning to be performing to get a special day, you might decide to add some added flourishes to your melody. In case a song includes a lot of words, you need to likely present the words first ahead of the tune.
You've to know it, to be ready to instruct a tune. Research the tune till you learn it forward and backward, until it becomes an integral part of you. Tune in to the song and singalong with it. And soon you may sing the complete element properly, practice singing the soprano element. Or slow down it to acquire a different feel or you might wish to speed up the song.