Hi Warren and welcome to the forums.

I think you're on the right lines with a MIDI keyboard - it looks to me like the easiest way to get started. This is what I suggest, though others may have different ideas.

First of all, just play about with melody on the keyboard until you have something recognisable. Then in BiaB select a style and tempo which might best suit your melody. Open the mixer window and mute all the instruments except the drums and use this as a click track to keep you in time. Vary the tempo as required until you're happy with the result.

Record the melody and quantise if necessary. Now start to experiment by adding a chord sequence and un-mute one or more of the other instruments. You can easily experiment by changing the chords a bar at a time until you get something working.

If you want to add additional tracks, save the file and open it in Realband, where you can add as many tracks as you want.

Hope this helps in some small way.

Cheers, ROG.