The easiest and most basic bassline there is in gospel music is the shout bassline. You can play this in any key on the bass and movements will be the same for every key. Here it is in Eb:
Eb (3rd string, 6th fret) play with pinky
G (4th string, 3rd fret) play with index finger
Ab (4th string, 4th fret) play with middle finger
A (4th string, 5th fret) play with ring finger
Bb (4th string, 6th fret) play with pinky
C (3rd string, 3rd fret) play with index finger
Db (3rd string, 4th fret) play with middle finger
D (3rd string, 5th fret) play with ring finger
To play in other keys follow the same fomat, but move your hand position up or down to find the right key. If you don't know the names of your frets or strings, just slide your finger up or down the strings until you find where the singer is singing, then play. It is best to know the names of your frets and strings prior to playing.