Do yourself an enormous favour and purchase Bona Drag first (a compliation), followed by Vauxhall and I (studio album). If you're partial to those two, and you will be, then expand to Viva Hate, Your Arsenal etc. I adore Viva Hate, so that one might not be a bad start either, although I'm terribly biased.
I would make it a high priority to purchase all the Smiths albums as well. Hatful of Hollow would make a nice start if you haven't heard any before, as would Louder Than Bombs or The World Won't Listen, which are all compilations.
I sincerely hope you enjoy the records!