... and I take it you don't have the chord symbols written in.
Think of each chord as being made up of notes from a scale. Basically, assuming the bass note is the first (or tonic) note, you can make a basic triad chord using the third and fifth note of the scale, as well as the first. For instance, if your bass note is a C, then your third and fifth will be E and G.
This is for a major triad (ie on a major scale). For a minor triad, you would have to flatten the third by a semitone.
You also have to consider all sorts of 7ths, 9ths, augmented and diminished chords, half of which I don't understand myself, let alone be able to explain to you.
Finally, the bass line for a song doesn't necessarily use the tonic note of the chord. Unless you're experienced enough to be able to 'hear' the right chord, then it's going to be a lot of trial and error, mostly error, I'm afraid.
Is the song well-known? What about telling us what it is and seeing if anyone can point you to a website with the chord chart on it?