Personally I'd start with Reaper Man, but they're all really good.
If you don't already know, Pratchett liked to build multiple ongoing series within the broader Discworld universe. Basically if Discworld is the MCU then within it you get your Captain America movies and your Iron Man movies and so on.
In the case of Discworld, the big ones to know about are:
- Rincewind - Incompetent, cowardly wizard stumbles through inadvertent adventures.
- Death - The Sandman style cosmic weirdness.
- The City Watch - Fantasy crime procedurals.
- The Witches - Witches dealing with darkly irreverent takes on classic horror tropes.
- Moist Von Lipwig - A conman gets strong armed into running civic institutions.
What you have acquired is the second book of the Death series (Reaper Man), the second book of the Witches (Wyrd Sisters), and two largely standalone books (Small Gods and Moving Pictures). The first two can both be read without reading the preceding novels in their respective series, but it might not hurt to try to track those down first. You'd be looking for Mort and Equal Rites respectively.
In general Discworld stands up fairly well to reading out of order, and you certainly should not try to read the whole thing chronologically (the first two books, especially, are pretty bad). The best approach is to pick a single sub series and read that in order. If you start with those four, based which you like best I'd continue with that series or a related one. Reaper Man or Wyrd Sisters, continue with that series. Moving Pictures, you'll probably want more stuff set in Ank Morpork like the city watch and the Moist Von Lipwig / "Industrial revolution" series. Small Gods is mostly its own thing, but you'll get more of that vibe with Rincewind.