Hey Rob,
Above I have written a Cminor scale, as a starting point. C, the first note, is the "root", it's the
note that you will generally want to start and end your loop with, just for a start. G, the fifth note,
is the "dominant", it's the note that tends to lead to the "root". So, let's start really simple, and
write a bassline that goes C, G, C. It probably doesn't sound that hot, but at least it will show
you how the dominant 'leads to' the tonic. Now let's write a melody or someting ok? Look at
your bass notes, there are specific notes that sound good with those notes in Cminor. On the
C's you'll prolly want to use some Eb's and G's (C, Eb, G is a "Cminor triad"). On the G's you'll
prolly want to use some B's and some D's (G, B, D is a "Gmajor triad") Other than that, just try
to stick to the notes in the scale (as written above, for now) and it should enable you to create a
melody that moves around a bit. Next, try doing the same thing, but put a few more bass notes
between the first C and the G (ie. C, Eb, D, F, G, C) and try writing melodies using what you
thing the triads for those notes may be. Hope that keeps you busy for a bit!
Peter