I would suggest that you read the actual Dark Tower books in the order in which they were published. You can then move on to the comics and the short story about the Little Sisters of Eluria.
It would be incredibly difficult and time consuming to read all of the related secondary works - that list includes the majority of King's books: It, Insomnia, The Stand, 'Salem's Lot, Hearts in Atlantis, The Mist, The Eyes of the Dragon, Everything's Eventual, Rose Madder, The Talisman, From a Buick 8, Firestarter, Dreamcatcher, Cujo, The Long Walk, Misery, The Dead Zone, and many, many more.
In my own personal opinion, of the secondary books that I have read (It, Insomnia, Misery, The Dead Zone, 'Salem's Lot, Eyes of the Dragon, Firestarter, The Long Walk, The Stand), it seems that the most important ones are as follows:
- The Stand
- The Eyes of the Dragon
- 'Salem's Lot
You won't miss many important points if you don't read any of these books, but they do help to provide some background information that enriches the story of the actual Dark Tower series.
The order in which you read the books is ultimately up to you as the reader. I imagine that many people would recommend reading the secondary stuff first, then starting on the Dark Tower books, comics, etc, but really, it doesn't make much difference.
The books of the Dark Tower series tend to jump back and forth between different timelines and the chronology is not particularly consistent. For example, most of book IV (Wizard and Glass) takes place in Roland's childhood; some of the events of book VIII (The Wind Through the Keyhole) take place between the end of book IV and the beginning of book V, but the main story is also set in Roland's youth. More generally, all of the books include passages in which Roland reminisces about the past and tells stories about what the world was like before it "moved on".
In short, the best advice I can offer you is to read the books in whatever order seems best to you. There is no right or wrong way to enjoy this epic saga. The only important thing is to immerse yourself in the story and become part of Roland's world.
Note: There really isn't a correct answer to this question. It is all a matter of opinion. As if to prove this point, here is a list compiled by someone else, showing their own recommended reading list and order in which he/she thinks they should be read. One of the most interesting features of this list is that it very cleverly suggests reading The Gunslinger (i.e., Book I) again after finishing the series - this suggestion makes a lot of sense to those of us who have read the series. However, it is very important that I point out that this list contains TWENTY FIVE BOOKS. This just goes to show you how much secondary material there is to read.
Another list, apparently written before the publication of The Wind Through the Keyhole (Book VIII), is slightly different from the previous one.