Well the Colour of Magic and the Light Fantastic are pretty good starting points for the Discworld seeing as they were the first. The writing quality is probably the lowest in all of Pratchett's works, but they're still a fun parody of the narrative and fantasy themes of the time and are a fun adventure.

I personally started on Jingo, one of the stories about Sam Vimes and it remains a favorite to this day.

The nice thing about Discworld is that each story is self contained enough that they don't need to be read in order for the most part. Though a lot of references are made to previous books in some form or another.

As for Terry's non-Discworld stuff, well I can't really say I recall much of it. The Bromeliad trilogy was nice but meant for younger audiences, he co-authored Good Omens which is great.

I'd say that you should just look through the synopsis of several books and pick the one that catches your eye and start there, then just read any of his books that sound good to you.