Glenda,

    This "be going to" construction has been written about very thoughtfully by Joan Bybee. It has gramamticalized fairly recently (since Shakespeare's time) from a construction for expressing movement toward a place, to a construction expressing intention, to a construction that expresses epistemic prediction. ("I am going to New York. I am going to write a novel. It is going to rain hard.) In those last two manifestations, it can act as a substitute for "will." The best way to analyze your example, i think, is as modal auxiliary for "write." When the construction is followed by a noun ("I am going to the store"), "to the store" functions as a prepositional phrase.

    It's interesting to know that all our modals have gramamticalized from lexical verbs, most of that during the period for which we have written records. Bybee uses this as a key part of her argument for seeing language as "a complex adaptive system." 

    Unlike "will," be going to can also convey past intention. ("I was going to pay my bills, but I ran out of money."

    Lanston Hughes' work makes for great classroom study since he uses nonstandard forms so thoughtfully and wisely.

Craig