There are many suggested reading orders out there (here's one and here another, plus a forum discussion on the topic). This is just one, and it's not for the faint of heart. Stephen King writes long books, and interspersing the main series with lots of slightly-but-not-very related behemoths is a good way to never finish the series. Most of the references to other works are found in the last three main novels, so perhaps between books IV and V is a nice stopgap for many of them. Another way to do it is to read all related works first, and then The Dark Tower. Yet another is to read The Dark Tower first, and then all related works. The references often work both ways.
Interested constant readers should of course also see Stephen King's official list of The Dark Tower connections for a more comprehensive list of the actual references found in the books on this list, as well as some more tenuous connections I don't mention.
First, the comics. Note that they are official and canon, even though they're not actually written by Stephen King per se – they're plotted by Robin Furth (King's personal research assistant and the author of the authorized Stephen King's The Dark Tower: A Complete Concordance), and King is Creative and Executive Director.
Chronologically, the comics serve as a prequel to the main series of novels. However, the first comic, The Gunslinger Born, is a re-telling of a flashback sequence that comprises the bulk of the fourth novel, Wizard and Glass . (The other comics are sequels to this one, and do not adapt directly from the novels.) You can read the comics before the main series of novels (if you're re-reading the series you might as well skip most of Wizard and Glass). Or they can all be read after Wizard and Glass. If I were reading The Dark Tower for the first time, reading the comics in the middle of the novel series kind of breaks the pace, although doing so sets up The Crimson King as more of a proper villain than King did in his novels, so it's not a bad idea actually.
- The Dark Tower: The Gunslinger Born (comics)
Like mentioned above, this is largely a re-telling of the bulk of Wizard and Glass. The following comics are direct sequels to this one, but tell the continuation of the story of Roland's youth and the fall of Gilead instead of a re-telling
- The Dark Tower: The Long Road Home (comics)
- The Dark Tower: Treachery (comics)
- The Dark Tower: The Fall of Gilead (comics)
- The Dark Tower: Battle of Jericho Hill (comics)
- The Dark Tower: The Journey Begins (comics)
Direct sequel to Battle of Jericho Hill, and prequel to The Gunslinger with slight overlap
And then the books (with main novel series in bold). This is a suggested, mostly chronological, order. Most of the related novels that are not in the main series can be read at any point before it is listed, but they're listed at the point I suggest reading them, and they should at least not be read later. If you're reading the series for the first time, reading all the related novels are a big endeavour, and if you get tired of it just march on with the main novels, I beg. They're the most important.
- The Dark Tower: The Sorcerer (comics)
Tells the backstory of The Man in Black aka Marten Broadcloak aka The Guy Who Pops Up In A Million Stephen King Stories
- The Eyes of the Dragon
Also tells of the (or a) backstory of The Man in Black. Can be read at any time, but fits here (and some of it is referenced lightly in The Drawing of the Three, so preferably before that).
- The Dark Tower: The Gunslinger (revised edition)
- The Dark Tower II: The Drawing of the Three
- Rose Madder
Not required reading, but some concepts introduced in The Drawing of the Three appear (ka), as well as a location revisited in The Waste Lands (Lud), so this seems like a good place to put this one.
- The Dark Tower III: The Waste Lands
- "The Mist"
Short story from the collection Skeleton Crew. Not required reading, but introduces a concept found in Wizard and Glass (thinnies, todash space).
- The Stand (complete and uncut edition)
Directly referenced in Wizard and Glass, features The Man in Black as well as some other tidbits and themes (ka, black and white, etc). Not strictly required, but very good.
- The Dark Tower IV: Wizard and Glass
Mostly consists of a flashback story from Roland's youth, which is also told on its own in the comic The Gunslinger Born.
- "The Little Sisters of Eluria"
A short story from the collection Everything's Eventual. Like the comics, it tells a story from Roland's life before The Gunslinger (but after the comic book The Gunslinger Born/the flashback in Wizard and Glass).
- The Dark Tower 4.5: The Wind Through the Keyhole
A frame story which fits chronologically between books IV and V, this one also consists of a flashback to Roland's youth but mostly a fairy tale; can really be read whenever
- 'Salem's Lot
Features one very important character (Father Callahan) who appears in Wolves of the Calla and later books, and features his backstory.
- Desperation and The Regulators
Introduce concepts (such as The Low Men, twinners, etc) found in the last three main novels. Not sure where to put it, but they contain references to "Little Sisters of Eluria" and The Talisman.
- From a Buick 8
Not required but contains references to The Low Men.
- Hearts in Atlantis
Introduces major characters in The Dark Tower (Ted Brautigan, the Crimson King, The Low Men, possibly the Man in Black).
- The Dark Tower V: Wolves of the Calla
A fair portion of this novel acts as a loose sequel to 'Salem's Lot mentioned above, and this portion also features The Low Men.
Not strictly speaking required reading (plus it's ungodly long), but it's a very good book and ties into the series in general (see the turtle of enormous girth!) and the next to main novels more specifically although still vaguely.
- The Dark Tower VI: Song of Susannah
- The Talisman and Black House
Take place here chronologically. Introduce concepts found in The Dark Tower; most notably it features The Crimson King. Lots of small references as well. Pretty much required.
Also features The Low Men.
Introduces another major character who appears in The Dark Tower (Patrick Danville), and features the Crimson King.
- "Everything's Eventual"
Another short story from the eponymous collection. Also introduces a character in The Dark Tower (Dinky Earnshaw).
- The Dark Tower VII: The Dark Tower
- The Dark Tower: The Gunslinger (original edition)
Just for completeness's sake... And perhaps an expanded view of the series as a whole.
Long days and pleasant nights.