First step: pray. Ask GOD to show you what you need to know. You don't need to play the song like anyone else, just give GOD the glory with the song.
I'm learning this myself, but so far what works the best for me is to learn the melody. When you can play the melody with one finger, then you know what key the song is in.
That is the second step: learn the melody.
Third step . . . chords. Now you know what key the song is in, so you know what chords will be the most common. Knowing what key the song is in is like needing to find something in a big house and at least narrowinging down which room the thing is in. So if you know the song is in Ab, then you know you can have (and probably will have) Db chords, Eb chords, F minor chords, C minor chords, G diminished chords, and Ab chords.
Fourth step . . . bassline for upbeat songs, progressions for ballads. Upbeat songs will need a basline to drive the rhythm, especially if you don't have a bass guitar. Gospel ballads don't usually have a bassline, they just use progressions and arpeggios.
Final step: Introduction, turnarounds, and ending. Work out an introduction that lets the people know what song you will be playing. Perfect any turnarounds and progressions. Then do your ending.
Hope this helps . . . :wink:
Be Blessed