Guitar or melodeon are good first instruments because the enjoyment that can be gained after learning a few simple tunes and basic chords can be a great motivator, requiring hours rather than months of practise to be able to start enjoying making 'real' music. However, both my children started on the violin and went through the usual pain barrier of learning all the scales, understanding sharps, flats and different keys . It was only when they started to learn the piano that suddenly everything slotted into place, simply because they could visualise what they were doing with relation to the keyboard, black notes, white notes, sharps, flats, etc.

Any instrument is a good one to start on if it's the one you want to learn but I would also recommend learning at least some basic piano at the same time.

Chris