All right - I'll assume you know nothing, and start there.

If you've got one bar in 4/4 time:

1...2...3...4...

You can start with a simple pattern of playing the root note of the chord on beat 1.

By "root note", I mean the note that's the first note in the scale the chord was taken from.

So if you've got a "C major" or "C minor" or "C7" or "Cmaj7", the root note is going to be a "C" note - the same name as the chord.

Here, the "C" under the 1 indicates the root of the chord is played on first beat of the bar:

1...2...3...4...
C...............

If the chord changed from a C on the first beat to a F on the third beat, play the bass of the new chord on the chord change:

1...2...3...4...
C.......F.......

If you start a new bar and the chord didn't change, play the bass of the current chord on the first beat.

So in this example, imagine the chord chart looks like this:

C / F / | F / / /

That is, two quarter note beats of a C major chord and two quarter note beats of a F major chord in bar 1, followed by 4 quarter note beats of an F major chord. On bass, you would play an "C" on beat 1, and an "F" one beat 3 in the first bar.

1...2...3...4....
C.......F........

in the next bar, the chord stays on an F major chord, so play an F on beats 1 and 3:

1...2...3...4....
F.......F........

Now, the next step up is to alternate the bass note between the root note and the fifth of the chord.

The "fifth of the chord" means the fifth note of the scale the chord belongs to. If the chord is C major, the C major scale is C,D,E,F,G,A,B, and the 5th note of the scale is "G".

So if you had a full bar of some sort of C chord, instead of playing another "C" note on beat 3, you could play "G", which is the fifth of the chord:

1...2...3...4...
C.......G.......

What if the chord was an F major chord? The scale F major has the notes F,G,A,Bb,C,D,E, so the fifth is a "C" note. So the alternate note would be a "C":

1...2...3...4....
F.......C........

This is going to give a sort of "oom-pah" feel to the music. So instead, you probably want to put the fifth on the 4th quarter beat, like so:

1...2...3...4...
C...........G...

Much cooler.

If the chord indicates an inversion, like C/E, or Dm7/G, this note under the slash indicates the note to play on the bass instead of the root note. So if the chord was C/E, you could play and "E" on the first beat. You could stay safe and play the "E" again on the third beat of the bar:

1...2...3...4...
E...........E...

Or get all fancy-schmancy and play the fifth of the chord (E major = E,F#,G#,A,B,C#,D#, so the fifth is "B"):

1...2...3...4...
E...........B...

If you don't know how to easily find the fifth of a chord, there's a chart you can Google called the "Circle of Fifths" that will show you the fifths of all the chords (and is useful for a lot of other music-theory related things).

And that's 90% of what you need to play bass.

Assuming you knew all that... what sort of additional information are you looking for?

-- David Cuny
My virtual singer development blog

Vocal control, you say. Never heard of it. Is that some kind of ProTools thing?