Ron, this can be done a number of ways.
I'd begin by choosing a backer board that is a true 1/2" thick to match up to the wallboard. Durock will, Hardibacker will not. Doing this will save you a number of headaches downstream. Then arrange your backerboard cuts so that the transition seam to wallboard will be completely covered by tile. Or, to put this a different way, have your tile overlap onto the wallboard by an inch or so. Tape and mud that transition just like the other backerbaord seams.
I'd recommend you tape and mud the seams as you set the tile, not before.
P.S. Why are you using greenboard?