II don't where to start and how to build melodies in DADGAD. Do you have any suggestions that would help me start with the open tunings again?
First off, as you're probably already aware, compared to standard, DADGAD is a dropped tuning so assuming we're in the key of D, the third and second strings are adjacent scale tones—G and A. This simple fact allows you to easily play scale fragments and melodies using combinations of fretting notes and open strings for harp like sounds
With the DADGAD tuning you can play awesome stuff with literally just one finger, (no kidding), and after a day or two, you should be able to switch between the standard tuning and DADGAD relatively easily.
The thing I like about this tuning is the fact that you can play most styles of music with it.. it's used frequently in Folk music but will work really well for Blues ,Funky stuff and things that have an Eastern type of feel to them.
Even though I'm not one of their greatest fans a good example of the DADGAD tuning is the Led Zeppelin track 'Kashmir'.. as the main riff of the guitar is playing John Paul Jones plays some very cool melodies within the progression and the music really feels like it's taking you somewhere,sonically..the riff is actually pretty basic but the counter melodies from Jones' keyboard have an almost orchestral feel to them.
With any alternate tuning I think it's just a simple case of trial and error and the more you experiment with them the more you'll discover what does or doesn't work within these non standard tunings.