Each book has a standalone plot, but characters develop significantly from book to book, so it’s worth reading the books in some order. There are several different sub-series that feature sets of characters, and those at least are worth reading in order of publication.
You could start from the beginning (starting with the colour of magic), which would help you get to grips with the world-building and introduce each character at the beginning of their story. However, Pratchett grew as a writer over time, so the first books are not his best (though colourful and clever). If you are willing to try a couple, maybe start with the colour of magic, but be prepared to pick up another book from the middle of the series if it turns out that one doesn’t do it for you.
Otherwise, you could try of the series within series, and start there - Wyrd Sisters, Mort, or Guards Guards, depending on which appeals. Pratchett was hitting his stride by the time he wrote those, but hadn’t peaked - the best is still to come, but you’ll appreciate it more for having followed the characters through their arcs.
Hogfather is a festive read in the run up to
Christmas, and I think it would stand alone quite well. If you still can’t decide where to start, try there :)