Maybe these chords will help get you started:
"Let It Rain" by Bishop Paul S. Morton
Key of C LH/RH
G,A,C,D / G,A,C,D
C / G-C-E Open the flood gates of
G / G-B-D Heaven
B / A-D-F Let
E / Ab-C-D-G it
A / A-C-E rain
A / A-C-E Let
G / G-B-D it
F / F-A-C rain
(repeat over & over)
That is basically the whole song. The guy playing the piano on the CD does a whole lot of improvising. So, just play around with it and try to put your little spin on it. If u need more help just ask for Trey.