There is no single right way into the Discworld — the books were written to be picked up in almost any order, and Pratchett rather liked it that way. But after 41 books, "where do I start?" is the most-asked question of all, and two routes work best.
If you want the short answer: pick a strand — a set of characters — and read just their books, in order. Each thread stands on its own and gets stronger as it goes. The Witches and the City Watch are the two most reliable places to begin. Or, if you'd rather see the whole Disc grow up in real time, take them in the order they were written.